https://eyewiki.org/w/api.php?action=feedcontributions&user=Shawn.Sell&feedformat=atomEyeWiki - User contributions [en]2022-12-03T09:48:26ZUser contributionsMediaWiki 1.35.8https://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=61465Cataract Surgery Following Radial Keratotomy2020-10-04T19:58:49Z<p>Shawn.Sell: /* Introduction: */ PERK study, symptoms/signs</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Assigned editor=Jordan.S.Masters<br />
|Date reviewed=August 4, 2020<br />
|Article status=Update Pending<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D. Between 6 months-2 years post-op, patient's averaged a +0.21 D/year hyperopic shift, but this rate decreased to 0.06 D/year by the 2-10 year post-op time frame. The significance of this study demonstrated that a hyperopic shift occurred throughout the duration of the entire 10 years after surgery, which should be accounted for during future surgical planning. A greater hyperopic shift was found in patients with smaller optical zones. Furthermore the PERK study demonstrated a diurnal fluctuation mean myopic shift 0.31 +/- 0.58 D from the morning to evening.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= '''Method:''' =<br />
<br />
=== Symptoms and Signs: ===<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incisions (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Diurnal fluctuation occurs as a result of hypoxia from lid closure during sleep which creates edema within the incisions and subsequent hyperopic shift upon awakening. This hypoxic edema resolves throughout the day, and by the evening causes on average a myopic shift of 0.31D (2). Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
=== Formulae: ===<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== Historical Method: ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== '''Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<nowiki/>''' ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam and Power for the Sagittal Curve (PWR_SF_Pupil_4.0 mm Zone: 4mm zone centered on pupil measuring Sagittal Curve mean K)<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
= '''Surgery:''' =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5). Otherwise once a surgical plan including type of anesthesia is in place anticipate any other intra-operative such as a 10-0 nylon should be on standby if wounds dehisce (see below).<br />
<br />
= '''Complications:''' =<br />
Intra-operative wound dehiscence can occur in very weak wounds under infusion pressure and lead to loss of stability and high fluid utilization. Suturing these may add some partial stability both for the cornea and chamber mechanics. Also control of infusion pressure and minimizing number of times in/out of the eye will be beneficial to minimize compression/decompression events.<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60527Cataract Surgery Following Radial Keratotomy2020-08-29T20:36:32Z<p>Shawn.Sell: /* Method: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= '''Method:''' =<br />
<br />
=== Symptoms and Signs: ===<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
=== Formulae: ===<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== Historical Method: ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== '''Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<nowiki/>''' ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam and Power for the Sagittal Curve (PWR_SF_Pupil_4.0 mm Zone: 4mm zone centered on pupil measuring Sagittal Curve mean K)<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
= '''Surgery:''' =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= '''Complications:''' =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60526Cataract Surgery Following Radial Keratotomy2020-08-29T20:35:42Z<p>Shawn.Sell: Undo revision 60525 by Shawn.Sell (talk)</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= '''Method:''' =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== Historical Method: ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== '''Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<nowiki/>''' ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam and Power for the Sagittal Curve (PWR_SF_Pupil_4.0 mm Zone: 4mm zone centered on pupil measuring Sagittal Curve mean K)<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
= '''Surgery:''' =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= '''Complications:''' =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60525Cataract Surgery Following Radial Keratotomy2020-08-29T20:35:10Z<p>Shawn.Sell: </p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
=== '''Background:''' ===<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
=== '''Radial Keratotomy:''' ===<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
=== '''Prospective Evaluation of Radial Keratotomy (PERK) study:''' ===<br />
This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
=== '''Challenges for IOL Calculation:''' ===<br />
RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= '''Method:''' =<br />
<br />
=== Symptoms and Signs ===<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
=== Formulae ===<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== Historical Method: ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== '''Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<nowiki/>''' ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam and Power for the Sagittal Curve (PWR_SF_Pupil_4.0 mm Zone: 4mm zone centered on pupil measuring Sagittal Curve mean K)<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
= '''Surgery:''' =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= '''Complications:''' =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60524Cataract Surgery Following Radial Keratotomy2020-08-29T20:32:31Z<p>Shawn.Sell: Undo revision 60523 by Shawn.Sell (talk)</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= '''Method:''' =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== Historical Method: ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== '''Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<nowiki/>''' ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam and Power for the Sagittal Curve (PWR_SF_Pupil_4.0 mm Zone: 4mm zone centered on pupil measuring Sagittal Curve mean K)<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
= '''Surgery:''' =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= '''Complications:''' =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60523Cataract Surgery Following Radial Keratotomy2020-08-29T20:29:23Z<p>Shawn.Sell: /* Method: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
== '''Introduction:''' ==<br />
<br />
=== '''Background:''' ===<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
=== '''Radial Keratotomy:''' ===<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
=== '''Prospective Evaluation of Radial Keratotomy (PERK) study:''' ===<br />
This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
=== '''Challenges for IOL Calculation:''' ===<br />
RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
== '''Method:''' ==<br />
<br />
=== Symptoms and Signs ===<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
=== Formulae ===<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== Historical Method: ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
'''Requirements:''' <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== '''Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<nowiki/>''' ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam and Power for the Sagittal Curve (PWR_SF_Pupil_4.0 mm Zone: 4mm zone centered on pupil measuring Sagittal Curve mean K)<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
== '''Surgery:''' ==<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
== '''Complications:''' ==<br />
<br />
=== Intra-operative ===<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
=== Post-operative: ===<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60522Cataract Surgery Following Radial Keratotomy2020-08-29T20:13:58Z<p>Shawn.Sell: /* 4) Potvin-Hill (Pentacam) */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= '''Method:''' =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== Historical Method: ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== '''Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<nowiki/>''' ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam and Power for the Sagittal Curve (PWR_SF_Pupil_4.0 mm Zone: 4mm zone centered on pupil measuring Sagittal Curve mean K)<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
= '''Surgery:''' =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= '''Complications:''' =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60520Cataract Surgery Following Radial Keratotomy2020-08-29T19:55:38Z<p>Shawn.Sell: /* Introduction: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= '''Method:''' =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== Historical Method: ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== '''Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<nowiki/>''' ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
= '''Surgery:''' =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= '''Complications:''' =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60519Cataract Surgery Following Radial Keratotomy2020-08-29T19:54:52Z<p>Shawn.Sell: /* Introduction: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= '''Method:''' =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== Historical Method: ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== '''Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<nowiki/>''' ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
= Surgery: =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= Complications: =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60518Cataract Surgery Following Radial Keratotomy2020-08-29T19:52:26Z<p>Shawn.Sell: /* https://iolcalc.ascrs.org/wbfrmCalculator3.aspx */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= Method: =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== Link: https://iolcalc.ascrs.org/wbfrmCalculator3.aspx ====<br />
<br />
===== 1) Effective Refractive Power (EyeSys Topography) =====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
===== 2) Average Central Power =====<br />
<br />
===== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) =====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
===== 4) Potvin-Hill (Pentacam) =====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam<br />
<br />
===== 5) DK-Holladay-IOLM (IOL Master or Lenstar) =====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
===== 6) OCT =====<br />
<br />
===== 7) '''Barrett True K''' =====<br />
<br />
= Surgery: =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= Complications: =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60516Cataract Surgery Following Radial Keratotomy2020-08-29T19:50:04Z<p>Shawn.Sell: /* Formula: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= Method: =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== Formula: ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== '''American Society of Cataract and Refractive Surgery: IOL power calculator for eyes with prior RK''' ===<br />
<br />
==== https://iolcalc.ascrs.org/wbfrmCalculator3.aspx ====<br />
<br />
==== 1) Effective Refractive Power (EyeSys Topography) ====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
==== 2) Average Central Power ====<br />
<br />
==== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) ====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
==== 4) Potvin-Hill (Pentacam) ====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam<br />
<br />
==== 5) DK-Holladay-IOLM (IOL Master or Lenstar) ====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 6) OCT ====<br />
<br />
==== 7) '''Barrett True K''' ====<br />
<br />
= Surgery: =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= Complications: =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60515Cataract Surgery Following Radial Keratotomy2020-08-29T19:47:57Z<p>Shawn.Sell: /* 3) Potvin-Hill (Requires use of Oculus Pentacam) */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= Method: =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
'''Double K Formulas:''' Utilize various algorithms (SRK/T, Holladay or Hoffer Q) based on '''K value''' and '''axial length''' to determine '''effective lens position (ELP)'''. Pre-RK Ks are used for correct ELP and post-RK Ks are used for corneal power.<br />
<br />
1) True K RK algorithm<br />
<br />
2) DK-Holladay-IOLM<br />
<br />
3) Potvin-Hill<br />
<br />
Among the Double K formulae True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). When no refractive history is available True K (No History) and unadjusted Haigis were most accurate.<br />
<br />
'''Haigis formula:''' ELP calculated using '''axial length''' and '''AC depth'''.<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== '''Formula:''' ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
-Modified Double-K (DK) formula using a presumed pre-RK K of 43.86 to calculate ELP and a post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
=== American Society of Cataract and Refractive Surgery: '''IOL power calculator for eyes with prior RK''' ===<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
==== 1) Effective Refractive Power (EyeSys Topography) ====<br />
-Effective Refractive Power (EffRP) used as the central corneal power by taking all points in the central 3mm and applying this for IOL power calculation.<br />
<br />
==== 2) Average Central Power ====<br />
<br />
==== 3) Atlas 1-4 (Zeiss Humphrey Atlas topographer) ====<br />
-Averages annular powers on the Numerical View at 1mm, 2mm, 3mm, and 4mm to calculate average corneal power <br />
<br />
==== 4) Potvin-Hill (Pentacam) ====<br />
-Holladay 1 IOL formula calculated using thinnest corneal pachymetry from the Pentacam<br />
<br />
==== 5) DK-Holladay-IOLM (IOL Master or Lenstar) ====<br />
-Modified double-K formula using post-RK K (from IOL master or Lenstar) to calculate corneal refractive power <br />
<br />
==== 6) OCT ====<br />
<br />
==== 7) '''Barrett True K''' ====<br />
<br />
= Surgery: =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= Complications: =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60514Cataract Surgery Following Radial Keratotomy2020-08-29T19:02:08Z<p>Shawn.Sell: /* Formula: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= Method: =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
IOL calculation may be performed using a third-generation formulas like Holladay 2. Alternatively the Double K correction method with formulas like SRK/T, Holladay or Hoffer Q can be utilized.<br />
<br />
Among the Double K formulae ( True K RK algorithm, DK-Holladay-IOLM (requires IOL Master), Potvin-Hill), True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). <br />
<br />
3) Potvin-Hill<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
==== '''Formula:''' ====<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
==== Example: ====<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
<u>I'''OL power calculator for eyes with prior RK: '''</u>American Society of Cataract and Refractive Surgery<br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
1) Effective Refractive Power (EyeSys Topography)<br />
<br />
2) Average Central Power<br />
<br />
3) Atlas 1-4 (Zeiss Humphrey Atlas topographer)<br />
<br />
4) Pentacam<br />
<br />
5) IOL Master (double-K modified 1 formula using a pre-op K of 43.86 D and post-op mean K from the IOLMaster or Lenstar)<br />
<br />
6) OCT<br />
<br />
7) '''Barrett True K'''<br />
<br />
= Surgery: =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= Complications: =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60512Cataract Surgery Following Radial Keratotomy2020-08-29T18:49:19Z<p>Shawn.Sell: /* Double K Formulae: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= Method: =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
IOL calculation may be performed using a third-generation formulas like Holladay 2. Alternatively the Double K correction method with formulas like SRK/T, Holladay or Hoffer Q can be utilized.<br />
<br />
Among the Double K formulae ( True K RK algorithm, DK-Holladay-IOLM (requires IOL Master), Potvin-Hill), True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). <br />
<br />
3) Potvin-Hill<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
=== Formula: ===<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
=== Example: ===<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
<u>I'''OL power calculator for eyes with prior RK: '''</u>American Society of Cataract and Refractive Surgery<br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
1) Effective Refractive Power (EyeSys Topography)<br />
<br />
2) Average Central Power<br />
<br />
3) Atlas 1-4 (Zeiss Humphrey Atlas topographer)<br />
<br />
4) Pentacam<br />
<br />
5) IOL Master (double-K modified 1 formula using a pre-op K of 43.86 D and post-op mean K from the IOLMaster or Lenstar)<br />
<br />
6) OCT<br />
<br />
7) '''Barrett True K'''<br />
<br />
= Surgery: =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= Complications: =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60511Cataract Surgery Following Radial Keratotomy2020-08-29T18:18:23Z<p>Shawn.Sell: /* Double K Formulae: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= Method: =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
IOL calculation may be performed using a third-generation formulas like Holladay 2. Alternatively the Double K correction method with formulas like SRK/T, Holladay or Hoffer Q can be utilized.<br />
<br />
Among the Double K formulae ( True K RK algorithm, DK-Holladay-IOLM (requires IOL Master), Potvin-Hill), True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). <br />
<br />
3) Potvin-Hill<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
=== Formula: ===<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
=== Example: ===<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
<br />
==== 1) '''True K Method''' ====<br />
<br />
==== 2) DK-Holladay-IOLM (requires IOL Master) ====<br />
<br />
==== 3) Potvin-Hill (Requires use of Oculus Pentacam) ====<br />
<u>Final IOL Power</u> = Holladay 1 IOL Calculation - 4.4554 + 0.0084 x CTmin<br />
<br />
CTmin: Minimum Central Thickness. Calculated by taking thinnest local pachymetry in microns from Oculus Pentacam Topometric Display<br />
<br />
<u>I'''OL power calculator for eyes with prior RK:'''</u><br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
<br />
<br />
= Surgery: =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= Complications: =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60510Cataract Surgery Following Radial Keratotomy2020-08-29T18:01:27Z<p>Shawn.Sell: /* Historical Method: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure (on the posterior cornea), it wasn't until the latter 20th century when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
= Method: =<br />
"Symptoms and Signs"<br />
<br />
Pre-operative assessment requires additional attention to details at all levels compared to standard cataracts surgery starting all the way in the beginning with the first assessment as described by a standard chief complaint and history of present illness. For eyes with numerous incision (e.g. 16 cut RK) fluctuation in vision can be a concerning symptom given the known effects of bio-mechanical weakening and secondary refractive effects that can be influenced by factors such as time of day, altitude and sleep cycles.<br />
<br />
For cases where vision fluctuates over the course of a 24 hour cycle, timing of when to perform exams and ancillary testing can be challenging. Some surgical cases go as far as warranting exams and measurements intentionally at different times of the day and detailed counseling on the refractive shifts and outcomes that will still be present after cataract treatment. Though some case series report limited success of stabilizing biomechanics of corneas with RK (7), such tailored control of wound healing and which areas preferentially tighten may be challenging as certain wounds could gape in comparison to non-incised corneal areas based on the amount of epithelial plugging.<br />
<br />
Given that cataracts only further aberrate post RK corneas, deciding the relative role of each on symptoms may be driven by the ability to perform diagnostic rigid contact lens fitting (particularly for new evaluations). For corneas with high number of incisions that result in very flat corneas, reverse geometry lenses (flatter in the center with some peripheral vaulting) may help in determine sources of decreased vision.<br />
<br />
"Formulae"<br />
<br />
IOL calculation may be performed using a third-generation formulas like Holladay 2. Alternatively the Double K correction method with formulas like SRK/T, Holladay or Hoffer Q can be utilized.<br />
<br />
Among the Double K formulae ( True K RK algorithm, DK-Holladay-IOLM (requires IOL Master), Potvin-Hill), True K method has been shown in some studies to have the lowest mean error especially when combined with partial history (8). <br />
<br />
3) Potvin-Hill<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. Of note this method may be confounded by progressive hyperopia after RK and myopic shift from nuclear sclerosis,. <br />
<br />
Requirements: <br />
<br />
1) Pre-RK Keratometry to calculate mean K <br />
<br />
2) Pre-RK Refraction <br />
<br />
3) Post-RK refraction <br />
<br />
=== Formula: ===<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
=== Example: ===<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D.<br />
<br />
=== Double K Formulae: ===<br />
1) '''True K Method'''<br />
<br />
2) DK-Holladay-IOLM (requires IOL Master)<br />
<br />
3) Potvin-Hill<br />
<br />
<u>I'''OL power calculator for eyes with prior RK:'''</u><br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
<br />
<br />
= Surgery: =<br />
Recent developments in imaging and devices have allowed for use of intra-operative technologies such as aberrometry and refractometry to adjunct assessment of IOL selection (5).<br />
<br />
= Complications: =<br />
Intra-operative<br />
<br />
Crossing if Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions(6), however clear corneal incisions are at a higher risk for further inducing irregular astigmatism, worsening of aberrations and loss of structural integrity if they cross the RK incisions.<br />
<br />
Post-operative:<br />
<br />
Standard risks of any intra-ocular surgery apply to this surgical scenario, however additional risks of refractive surprise and fluctuating vision due to altered corneal physiology and anatomy still apply. General risks related to this must be discussed in the pre-operative counseling (see above).<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534<br />
<br />
5. Curado, Sebastiao et. al. "Intraoperative Aberrometry Versus Preoperative Biometry for IOL Power Selection After Radial Keratotomy: A Prospective Study" J. Refractive Surgery. 2019. Oct 1;35(10):656-661.<br />
<br />
6. Meduri, A et. al. "Cataract surgery on post radial keratotomy patients" Int. J. Ophthalmol. 2017 Jul 18;10(7):1168-1170. doi: 10.18240/ijo.2017.07.12. eCollection 2107.<br />
<br />
7. Elbaz, Uri et. al. "Collagen crosslinking after radial keratotomy" Cornea. 2014 Feb;33(2):131-6. doi. 10.1097/ICO.0000000000000044.<br />
<br />
8. Turnbull, Andrew M.J. et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology, Volume 127, Issue 1, 45 - 51</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60175Cataract Surgery Following Radial Keratotomy2020-08-16T21:01:49Z<p>Shawn.Sell: /* Formula: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure, it wasn't until the 1980s when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
<br />
<br />
= Method: =<br />
IOL calculation should be performed using a third-generation formulas like Holladay 2. Alternatively the Double K correction method with formulas like SRK/T, Holladay or Hoffer Q can be utilized.<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. <br />
<br />
Requirements: <br />
<br />
-Pre-RK Keratometry to calculate mean K, Pre-RK Refraction, and Post-RK refraction <br />
<br />
=== Formula: ===<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (Change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
=== Example: ===<br />
<u>'''Pre-RK Keratometry Mean K:'''</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
'''<u>Change in spherical equivalent at corneal plane</u>'''<u>:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D. <br />
<br />
<u>I'''OL power calculator for eyes with prior RK:'''</u><br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
= Complications: =<br />
Crossing Limbal Relaxing Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions, however clear corneal incisions are at a higher risk for inducing irregular astigmatism if they cross the RK incision.<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60174Cataract Surgery Following Radial Keratotomy2020-08-16T20:59:58Z<p>Shawn.Sell: /* Method: */ Expansion of method section with historical method + examples of calculations</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure, it wasn't until the 1980s when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
<br />
<br />
= Method: =<br />
IOL calculation should be performed using a third-generation formulas like Holladay 2. Alternatively the Double K correction method with formulas like SRK/T, Holladay or Hoffer Q can be utilized.<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available. <br />
<br />
Requirements: <br />
<br />
-Pre-RK Keratometry to calculate mean K, Pre-RK Refraction, and Post-RK refraction <br />
<br />
=== Formula: ===<br />
'''Effective Corneal Power (Post-RK) = (Pre-RK Mean K) - (change in spherical equivalent at corneal plane)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. This is calculated by taking the difference between the Pre-RK refraction and Post-RK refraction and converting to the power at the corneal plane. The effective corneal power can now be used for the K value in IOL calculations. <br />
<br />
=== Example: ===<br />
<u>Pre-RK Keratometry Mean K:</u> Steep K (45D ) + Flat K (43D) = '''Mean K is (44D)''' <br />
<br />
<u>Myopic Correction corrected at corneal plane:</u> Pre-RK refraction (-6.5D), Post-RK refraction (-0.5D). The surgical correction is '''6 D''' at the spectacle plane. Correcting for a presumed vertex distance of 12mm using the vertex formula: Fc = F / (1-xF) <br />
<br />
Fc (Power at corneal plane) <br />
<br />
F(Power at spectacle plane) <br />
<br />
x (change in vertex distance in meters) <br />
<br />
Fc = 6/ (1 - 0.012 x 6) = 5.6 D or ~5.5 D at the corneal plane <br />
<br />
Alternatively you can estimate the change by adding 0.25 D for 2D change in power after 4D: <br />
<br />
{| class="wikitable"<br />
|4.00 D +0.25 D<br />
|-<br />
|'''6.00 D +0.50 D'''<br />
|-<br />
|8.00 D +0.75 D<br />
|-<br />
|10.00 D +1.00 D<br />
|-<br />
|12.00 D +1.25 D<br />
|-<br />
|14.00 D +1.50 D<br />
|}<br />
<br />
<br />
<u>Effective Corneal Power:</u> 44D - 5.5 D = 38.5 D. <br />
<br />
<u>I'''OL power calculator for eyes with prior RK:'''</u><br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
= Complications: =<br />
Crossing Limbal Relaxing Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions, however clear corneal incisions are at a higher risk for inducing irregular astigmatism if they cross the RK incision.<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60171Cataract Surgery Following Radial Keratotomy2020-08-16T18:02:51Z<p>Shawn.Sell: /* Introduction: */</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure, it wasn't until the 1980s when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopic patients with a refractive error ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of post-op patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise. Central corneal power is not the real corneal power required for IOL calculations. Instead we need to find the effective corneal power.<br />
<br />
<br />
<br />
= Method: =<br />
IOL calculation should be performed using a third-generation formulas like Holladay 2. Alternatively the Double K correction method with formulas like SRK/T, Holladay or Hoffer Q can be utilized.<br />
<br />
=== '''Historical Method:''' ===<br />
This method is most useful when pre-RK refraction and keratometry readings are available and the change in spherical equivalent. <br />
<br />
'''(Pre-RK Mean K) - (myopic correction)''' <br />
<br />
Myopic Correction: This value should ideally be taken a few years after RK stabilizes, however before myopia is induced by cataract formation. <br />
<br />
<u>I'''OL power calculator for eyes with prior RK:'''</u><br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
= Complications: =<br />
Crossing Limbal Relaxing Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions, however clear corneal incisions are at a higher risk for inducing irregular astigmatism if they cross the RK incision.<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=User:Shawn.Sell&diff=60110User:Shawn.Sell2020-08-14T00:29:09Z<p>Shawn.Sell: </p>
<hr />
<div>{{User<br />
|Approved=Yes<br />
|Name=Shawn Sell, MD<br />
|Location=Norfolk, Virginia<br />
|Affiliation=Eastern Virginia Medical School<br />
|Financial disclosures=None<br />
|Contact=sellss@evms.edu<br />
|Ophthalmologist=Yes<br />
|Subspecialty=Ophthalmology Resident<br />
|Degree type=M.D.<br />
}}</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60109Cataract Surgery Following Radial Keratotomy2020-08-14T00:22:38Z<p>Shawn.Sell: </p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure, it wasn't until the 1980s when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopia patients with a refractive error for ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of the patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:''' RK changes the curvature of the central cornea and it’s overall topography where the cornea is flat centrally and steeper in the periphery. Standard topography can overestimate central corneal power which results in a post-operative hyperopic surprise.<br />
<br />
<br />
<br />
= Method: =<br />
IOL calculation should be performed using a third-generation formulas like Holladay 2. Alternatively the Double K correction method with formulas like SRK/T, Holladay or Hoffer Q can be utilized.<br />
<br />
'''Historical Method:''' This method is most useful when pre-RK refraction and keratometry readings are available and the change in spherical equivalent. <br />
<br />
<u>I'''OL power calculator for eyes with prior RK:'''</u><br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
= Complications: =<br />
Crossing Limbal Relaxing Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions, however clear corneal incisions are at a higher risk for inducing irregular astigmatism if they cross the RK incision.<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60107Cataract Surgery Following Radial Keratotomy2020-08-13T23:25:15Z<p>Shawn.Sell: Additional Categories Added: Method. Expanded Introduction Section.</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
<br />
= '''Introduction:''' =<br />
<br />
'''Background'''<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
'''Radial Keratotomy'''<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure, it wasn't until the 1980s when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
'''Prospective Evaluation of Radial Keratotomy (PERK) study:''' This study was performed in 1982 and inclusion criteria was for myopia patients with a refractive error for ranging from -2.00D to -8.75 D. At 10 years the PERK study demonstrated that 43% of the patients between 6 months to 10 years developed a hyperopic shift of at least 1.00D.<br />
<br />
'''Challenges for IOL Calculation:'''<br />
<br />
= Method: =<br />
IOL calculation should be performed using a third-generation formulas (Holladay 2, SRK/T)<br />
<br />
'''Historical Method:''' This method is most useful when pre and post-RK keratometry values are available. <br />
<br />
<u>I'''OL power calculator for eyes with prior RK:'''</u><br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
= Complications: =<br />
Crossing Limbal Relaxing Incisions - Scleral tunnel incisions are typically preferred for wound construction. Alternatively clear corneal incisions can be made between between RK incisions, however clear corneal incisions are at a higher risk for inducing irregular astigmatism if they cross the RK incision.<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=Cataract_Surgery_Following_Radial_Keratotomy&diff=60106Cataract Surgery Following Radial Keratotomy2020-08-13T22:44:28Z<p>Shawn.Sell: Added IOL Calculator</p>
<hr />
<div>{{Article<br />
|Authors=Hoon.C.Jung,Rachel.dandar<br />
|Category=Cataract/Anterior Segment<br />
|Date reviewed=August 4, 2020<br />
|Meta description=Cataract surgery in the context of prior radial keratotomy presents unique challenges in peri-operative planning, surgical execution and refractive expectations.<br />
}}<br />
Background<br />
Cataract surgery in the context of prior radial keratotomy (RK) presents challenges in peri-operative planning, surgical execution and refractive expectations. An understanding of cornea biomechanics, wound healing, refractive power and complex biometric assessment is required to elevate the chances of safe surgical success and satisfactory outcomes.<br />
<br />
Cataracts describe a multi-factorial change to the eye's lens system inducing optical changes, visual degradation and effect on quality of life among other secondary effects. This pathology remains a continued significant burden to the world population even in modern times (3).<br />
<br />
Radial Keratotomy<br />
Though the concept of radial keratotomy for altering corneal biomechanics and refractive state has approached near a century since Sato's (1) publication of the described procedure, it wasn't until the 1980s when the popularity of the procedure took off being performed on the order of millions worldwide extending into the 1990s as well. As scientific analysis and clinical studies continued to accrue (2), the popularity of RK as a primary refractive treatment has since waned with procedures using LASER in different forms (excimer and femtosecond) and lens based treatments taking the forefront in the refractive surgery arena. As the patient population that had received RK aged, treatment of senescent processes, particularly cataract, became a meaningfully challenging venture reinvigorating the need to understand cornea physiology, pathology, wound healing and optics became evident as traditional assessments and methods of treatment had unsatisfactory results.<br />
<br />
<br />
<br />
<u>I'''OL power calculator for eyes with prior RK:'''</u><br />
<br />
https://iolcalc.ascrs.org/wbfrmCalculator3.aspx<br />
<br />
1. Sato, T (1939). "Treatment of conical cornea (incision of Descemet's membrane)". Acta Soc Ophthalmol Jpn (in Japanese). 43: 544–55.<br />
<br />
2. Waring GO, Lynn MJ, McDonnell PJ (October 1994). "Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery". Arch. Ophthalmol. 112 (10): 1298–308. doi:10.1001/archopht.1994.01090220048022. PMID 7945032.<br />
<br />
3. Khairallah, M, Vision Loss Expert Group of the Global Burden of Disease Study. Invest. Ophthalmol. Vis. Sci. 2015. Oct;56(11):6762-9. doi. 10.1167/iovs.15-17201.<br />
<br />
4. Geggel HS. Intraocular Lens Power Selection after Radial Keratotomy: Topography, Manual and IOLMaster Keratometry Results Using Haigis Formulas. Ophthalmology. 2015 May; 122(5): 897-902.doi. 10.1016/j.ophtha.2014.12.002.Epub 2015 Jan 2017. PMID: 25601534</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=User:Shawn.Sell&diff=60098User:Shawn.Sell2020-08-13T00:27:21Z<p>Shawn.Sell: </p>
<hr />
<div>{{User<br />
|Name=Shawn Sell, MD<br />
|Location=Norfolk, Virginia<br />
|Affiliation=Eastern Virginia Medical School<br />
|Financial disclosures=None<br />
|Contact=sell.shawn1@gmail.com<br />
|Ophthalmologist=Yes<br />
|Subspecialty=Ophthalmology Resident<br />
|Degree type=M.D.<br />
}}</div>Shawn.Sellhttps://eyewiki.org/w/index.php?title=User:Shawn.Sell&diff=60097User:Shawn.Sell2020-08-13T00:26:42Z<p>Shawn.Sell: Created page with "{{User |Name=Shawn Sell, MD |Location=Norfolk, Virginia |Affiliation=Eastern Virginia Medical School |Financial disclosures=None |Contact=sell.shawn1@gmail.com |Ophthalmologis..."</p>
<hr />
<div>{{User<br />
|Name=Shawn Sell, MD<br />
|Location=Norfolk, Virginia<br />
|Affiliation=Eastern Virginia Medical School<br />
|Financial disclosures=None<br />
|Contact=sell.shawn1@gmail.com<br />
|Ophthalmologist=Yes<br />
|Subspecialty=Ophthalmology Resident<br />
|Degree type=M.D.<br />
}}<br />
I'm an Ophthalmology Resident at Eastern Virginia Medical School. I'm working on an article with Dr. Hoon Jung.</div>Shawn.Sell