**doctor-hill.com**
**Determining Corneal Power**
**Following LASIK and PRK**
The following information contains an outline of several popular IOL power calculation methods that can be used following the various ablative forms of keratorefractive surgery for myopia, such as LASIK and PRK.
IOL power calculations following keratorefractive surgery should not be carried out using standard keratometry combined with any one of several popular 2-variable third generation theoretic formulas, such as SRK/T without a special correction. Instead, formulas with special adaptations, such the Holladay 2 formula (contained in the Holladay IOL Consultant), more modern regression techniques, or the myopic surgery specific Haigis-L formula (contained within the newest software release of the IOLMaster) should be used in this setting. To date, our office has had the best overall success with the combination of the Holladay Equivalent K feature of the Pentacam and the Holladay 2 formula contained within the Holladay IOL Consultant.
Basic information about the Holladay Equivalent K feature of the Oculus Pentacam can be found at:
http://www.oculususa.com/prd_comp.php
If your office does not have the Holladay IOL Consultant software package, a trial version can be downloaded from the Internet at:
http://www.docholladay.com/iolprogram.html
A major shortcoming of most 3^{rd} generation, 2-variable formulas, such as SRK/T, is that they often assume that the anterior and posterior segments of the eye are mostly proportional and use only the axial length and keratometric corneal power to estimate the postoperative location of the IOL, known as the effective lens position (ELPo).
Unless a specific correction is made for this situation, the artifact of centrally flattened Ks following keratorefractive surgery will have these formulas assume a falsely shallow post-operative ELPo.^{5} The end result is that without a special correction, following LASIK these formulas will typically recommend less IOL power than is actually required. This is a second, and little recognized, source of unanticipated post-operative hyperopia following keratorefractive surgery for myopia.
The SRK/T, Hoffer Q and Holladay 1 formulas can be used with caution, but it must be in conjunction with what has been termed an Aramberri double K method correction.^{5} You can review a post-keratorefractive surgery “Aramberri double K method” IOL power correction table based on recent literature at the following Internet location:
__www.doctor-hill.com/iol-main/prior-keratorefractive.htm__
Using the Holladay 2 formula, or a special “Aramberri double K method” correction for the SRK/T, Hoffer Q, or Holladay 1 formulas, you and your staff should now be able to do these calculations.
For the sake of illustration, we will run through several methods of central corneal power estimation and IOL power calculation using the data from a patient recently seen in our office.
If you are not yet experienced in doing this type of calculation, it is recommended that you read through this entire document before beginning.
**Clinical history method **^{3,4}
The clinical history method for corneal power estimation was first described by Holladay and later by Hoffer as:
*Kp* + *Rp* - *Ra = Ka* Where …
*Kp* = the average keratometry power before keratorefractive surgery, *and* …
*Rp* = the spherical equivalent before keratorefractive surgery, *and* …
*Ra* = the stable spherical equivalent after keratorefractive surgery, *then *…
*Ka* = the estimate of the central corneal power after keratorefractive surgery.
Central corneal power by keratometry will be referred to in *diopters* (D), even though it is better termed *keratometric diopters*. This is due to the fact that the cornea has a different index of refraction than keratometers, or corneal topographers (1.3333 *vs.* 1.3375).^{7}
Corrected for an estimated vertex distance of 12 mm, the historical estimation of the central corneal power of the right eye following LASIK would be carried out as follows:
If the Ks before LASIK were 45.37 D / 46.00 D, *and* …
The refraction before LASIK was -6.00 +0.75 x 135, *and* …
The stable refraction after LASIK was -0.25 sphere, *then* …
* Kp* + *Rp* - *Ra = Ka*
(45.69) + (-5.27) - (-0.25) = 40.67 D
Corrected for an estimated vertex distance of 12 mm, the historical estimation of the central corneal power of the left eye following LASIK would be carried out as follows:
If the Ks before LASIK were 45.25 D / 46.12 D, *and* …
The refraction before LASIK was -5.50 +0.75 x 060, *and* …
The stable refraction after LASIK was -1.25 +0.25 x 070, *then* …
* Kp* + *Rp* - *Ra = Ka*
(45.69) + (-4.83) - (-1.11) = 41.97 D
If you are unfamiliar with how to correct for vertex distance, click here to download an Excel spreadsheet that you can use to do this calculation. Be sure to maintain the correct sign (+ or -) when entering the refractive data, or the result will not be accurate.
Using this technique for estimating central corneal power, the following IOL powers are recommended by the Holladay 2 formula:
**SN60WF Target Refraction **
**Right** +17.00 D Plano
+17.50 D -0.38 D
**Left** +16.00 D Plano
+16.50 D -0.38 D
**Feiz and Mannis IOL power adjustment method** ^{1}
Another method that I like to use as an aid in determining the overall refractive picture when good historical data is available is the IOL power adjustment method of Feiz and Mannis. This is the method that is least likely to result in a post-operative hyperopic surprise. As you will see on the last page, we can use this method to set the upper limit of possible IOL powers as the Feiz and Mannis method will frequently produce a myopic over-correction.
Using this technique, first the IOL power is calculated using the pre-LASIK keratometry as though the patient had never undergone keratorefractive surgery. There is no special formula correction that needs to be employed for the Feiz and Mannis method. This calculated pre-LASIK IOL power is then increased by the amount of refractive change at the spectacle plane divided by 0.7. This approach is outlined as follows:
*IOLpre* -* (∆D *0.7) = *IOLpost *Where …
* IOLpre* = the power of the IOL using pre-LASIK keratometry, *and* …
* ∆D* = the stable refractive change after LASIK at the spectacle plane, *then …*
* IOLpost* = the estimated power of the IOL to be implanted following LASIK.
The Feiz and Mannis IOL power adjustment method for the right eye following LASIK would be carried out as follows:
If the calculated IOL power before LASIK is +9.18 D, *and* …
The change in refractive power at the spectacle plane is -5.38 D, *then* …
*IOLpre* - (∆D / 0.7) = *IOLpost*
+9.18 D - (-5.38 / 0.7) = +16.87 D
The Feiz and Mannis IOL power adjustment method for the left eye following LASIK would be carried out as follows:
If the calculated IOL power before LASIK is +10.27 D, *and* …
The change in refractive power at the spectacle plane is -4.00 D, *then* …
*IOLpre* - (∆D / 0.7) = *IOLpost*
+10.27 D - (-4.00 / 0.7) = +15.98 D
Using this technique, the following IOL powers are recommended:
**SN60WF Target Refraction **
**Right** +17.00 D Plano
+17.50 D -0.38 D
**Left** +16.00 D Plano
+16.50 D -0.38 D
**Modified Maloney method **^{2, 9}
Another method of post-LASIK corneal power estimation is one that was originally described by the well-known refractive surgeon Robert Maloney and subsequently modified by Doug Koch and Li Wang.
The advantage of this method is that it requires no historical data and has been reported to have a low variance when used with either the Holliday 2 formula or a 2-variable formula combined with an Aramberri double K method correction nomogram published by Koch and Wang.^{2}
Using this technique, the central corneal power is estimated by placing the cursor at the exact center of the Axial Map of the Zeiss Humphrey Atlas topographer. This value is then converted back to the anterior corneal power by multiplying the Axial Map central topographic corneal power by 376.0/337.5, which is the same as 1.114. An assumed posterior corneal power of 6.1 D is then subtracted from this product.
(*C*_{CP} x 1.114) - 6.1 = *Post-LASIK adjusted corneal power *Where …
* C*_{CP} = the corneal power with the cursor in the center of the Axial Map
of the Zeiss Humphrey Atlas topographer.
The Modified Maloney method for the right eye following LASIK is carried out as follows:
If the Axial Map central corneal power is 40.91 D, *then *…
* *(*C*_{CP} x 1.114) - 6.1 = *Post-LASIK adjusted corneal power*
(40.91 D x 1.114) - 6.1 = 39.47 D
The Modified Maloney method for the left eye following LASIK is carried out as follows:
If the Axial Map central corneal power is 41.36 D, *then *…
* *(*C*_{CP} x 1.114) - 6.1 = *Post-LASIK adjusted corneal power*
(41.36 D x 1.114) - 6.1 = 39.98 D
Using this technique for estimating central corneal power, the following IOL powers are recommended by the Holladay 2 formula:
**SN60WF Target Refraction **
**Right** +16.50 D Plano
+17.00 D -0.38 D
**Left** +17.00 D Plano
+17.50 D -0.38 D
**Masket method **^{10}
Another useful method of post-LASIK corneal power estimation is a postoperative regression method developed by Samuel Masket and recently published in the *Journal of Cataract and Refractive Surgery*. This technique takes advantage of the fact that there appears to be a linear relationship between the spherical equivalent of the total amount of the stable laser vision correction (LSE) and the over-estimation of central corneal power by simulated keratometry. This works following both myopic and hyperopic LASIK.
By this method, the IOL power is calculated using the Holladay 1 formula for axial lengths greater than 23.0 mm and the Hoffer Q formula for axial lengths less than 23.0 mm. The SRK/T formula is generally not recommended here as the artifact of very flat Ks may sometimes result in an under-correction. The IOL power is calculated without a double K method correction. Ks are provided by simulated keratometry. The calculated IOL power is then adjusted by the vertex distance corrected laser vision correction spherical equivalent determined at four to six months after the procedure, multiplied by -0.326 with 0.101 added to this product. Click here for an Excel spreadsheet to correct for vertex distance.
(LSE x -0.326) + 0.101 = *Post-LASIK IOL power adjustment *Where …
LSE = The vertex distance corrected laser vision correction spherical equivalent.
The Masket method for the right eye following LASIK would be carried out as follows:
If the calculated IOL power is +15.26 D for a -0.25 D result, *and *…
The stable laser vision correction spherical equivalent is -5.38 D* then *…
(LSE x -0.326) + 0.101 = *Post-LASIK IOL power adjustment*
(-5.05 D x -0.326) + 0.101 = +1.75 D =* IOL power adjustment*
+1.75 D + 15.26 D = 17.01 D = *Final adjusted IOL power*
The Masket method for the left eye following LASIK would be carried out as follows:
If the calculated IOL power is +15.09 D for a -0.25 D result, *and *…
The stable laser vision correction spherical equivalent is -4.00 D* then *…
(LSE x -0.326) + 0.101 = *Post-LASIK IOL power adjustment*
(-3.82 D x -0.326) + 0.101 = +1.35 D =* IOL power adjustment*
+1.35 D + 15.09 D = 16.44 D = *Final adjusted IOL power*
Using this technique for estimating central corneal power, the following IOL powers are recommended:
**SN60WF Target Refraction**
**Right** +17.00 D Plano
+17.50 D -0.38 D
**Left** +16.50 D Plano
+17.00 D -0.38 D
**Modified Masket method **^{10, 12}
During the process of validating the original Masket method, our data produced a slightly different regression formula. As with the Masket method, this technique takes advantage of the fact that there appears to be a linear relationship between the spherical equivalent of the total amount of the stable laser vision correction (LSE) and the over-estimation of central corneal power by simulated keratometry. This works for myopic and hyperopic LASIK.
By this method, the IOL power is calculated using the Holladay 1 formula for axial lengths greater than 23.0 mm and the Hoffer Q formula for axial lengths less than 23.0 mm. The SRK/T formula is generally not recommended as the artifact of very flat Ks may sometimes result in an under-correction. Ks are provided by simulated keratometry. The IOL power is calculated without a double K method correction. Ks are provided by simulated keratometry. The calculated IOL power is then adjusted by the vertex distance corrected laser vision correction spherical equivalent determined at four to six months after the procedure. It is then multiplied by -0.4385 and 0.0295 added to this product. Click here for an Excel spreadsheet that you can use to correct for vertex distance.
(LSE x -0.4385) + 0.0295 = *Post-LASIK IOL power adjustment *Where …
LSE = The vertex distance corrected laser vision correction spherical equivalent.
The modified Masket method for the right eye would be carried out as follows:
If the calculated IOL power is +15.26 D for a -0.25 D result, *and *…
The stable laser vision correction spherical equivalent is -5.38 D* then *…
(LSE x -0.4385) + 0.0295 = *Post-LASIK IOL power adjustment*
(-5.05 D x -0.4385) + 0.0295 = +2.25 D =* IOL power adjustment*
+2.25 D + 15.30 D = 17.55 D = *Final adjusted IOL power*
The modified Masket method for the left eye would be carried out as follows:
If the calculated IOL power is +15.09 D for a -0.25 D result, *and *…
The stable laser vision correction spherical equivalent is -4.00 D* then *…
(LSE x -0.4385) + 0.0295 = *Post-LASIK IOL power adjustment*
(-3.82 D x -0.4385) + 0.0295 = +1.71 D =* IOL power adjustment*
+1.71 D + 15.09 D = 16.80 D = *Final adjusted IOL power*
Using this technique for estimating central corneal power, the following IOL powers are recommended:
**SN60WF Target Refraction **
**Right** +17.50 D Plano
+18.00 D -0.38 D
**Left** +17.00 D Plano
+17.50 D -0.38 D
**Topographic central corneal power adjustment method **^{2, 9, 11}
Another method of post-LASIK corneal power estimation was originated by Doug Koch and Li Wang and is based on determining the central power of the cornea using either the Zeiss Humphrey Atlas topographer or the adjusted effective refractive power (EffRP_{adj}) of the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System.
Using this technique, the 1, 2, 3 and 4 mm power values of the Numerical View of the Zeiss Humphrey Atlas topographer are averaged together and used as the central corneal power value (Ccp). As an alternative, the adjusted effective refractive power (EffRP_{adj}) of the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System can be used. This value is then reduced by 19% for every diopter of myopia corrected by LASIK. We have added the 3 mm and the 4 mm power values from the Numerical View of the Zeiss Humphrey Atlas topographer to this calculation based on the fact that Holladay has found that sampling the 4.0 mm central cornea for the post-LASIK eye has returned very good results with the Oculus Pentacam.^{11} Our own clinical results have also shown an increase in accuracy by doing this.
*C*_{CP}* *-* (∆D *x 0.19) = *Post-LASIK adjusted corneal power *Where …
* C*_{CP} = the EffRP_{adj}, or the averaged Zeiss Atlas central corneal power, *and* …
* ∆D* = the refractive change after LASIK at the spectacle plane.
The topographic central corneal power adjustment method for the right eye would be:
If the averaged topographic central corneal power is 41.97 D, *and* …
The change in refractive power at the spectacle plane is -5.38 D, *then* …
*C*_{CP}* *+* (∆D *x 0.19) = *Post-LASIK adjusted corneal power*
41.97 D + (-5.38 D x 0.19) = 41.95 D
The topographic central corneal power adjustment method for the left eye would be:
If the averaged topographic central corneal power is 42.61 D, *and* …
The change in refractive power at the spectacle plane is -4.00 D, *then* …
*C*_{CP}* *+* (∆D *x 0.19) = *Post-LASIK adjusted corneal power*
42.61 D + (-4.00 D x 0.19) = 41.85 D
Using this technique for estimating central corneal power, the following IOL powers are recommended by the Holladay 2 formula:
**SN60WF Target Refraction**
**Right** +16.50 D Plano
+17.00 D -0.38 D
**Left** +16.00 D Plano
+16.50 D -0.38 D
**Corneal bypass method **^{ 14}
Another method of post-LASIK corneal power estimation is one that was recently described by Walter et al in the March, 2006 issue of the *Journal of Cataract and Refractive Surgery*. The advantage of this method is that it is done without having to calculate the post-LASIK corneal power.
Using this technique, the IOL power is calculated using the post-LASIK axial length and the pre-LASIK keratometry. The target refraction is set for the pre-LASIK spherical equivalent. This “bypasses” the post-LASIK corneal power.
The corneal bypass method for the right eye would be carried out as follows using the post-LASIK axial length:
If the pre-LASIK keratometry was 45.37 D / 46.00 D, and...
The spherical equivalent before LASIK was -5.62 D, and ...
The post-LASIK IOLMaster axial length is 26.32 mm, then ...
The IOL power for a -5.62 D refractive result would be +17.27 D
The corneal bypass method for the left eye would be carried out as follows using the post-LASIK axial length:
If the pre-LASIK keratometry was 45.25 D / 46.12 D, and...
The spherical equivalent before LASIK was -5.12 D, and ...
The post-LASIK IOLMaster axial length is 25.95 mm, then ...
The IOL power for a -5.12 D refractive result would be +18.23 D
Using this technique for estimating central corneal power, the following IOL powers are recommended by the Holladay 2 formula:
**SN60WF Target Refraction **
**Right** +17.50 D Plano
+18.00 D -0.38 D
**Left** +18.50 D Plano
+19.00 D -0.38 D
**Oculus Pentacam **^{12, 13}
The final and most useful method uses the Oculus Pentacam which images the anterior segment of the eye by a rotating Scheimpflug camera for 360°, generating an anterior segment picture in three dimensions. 25,000 corneal elevation points are measured and the center of the cornea is measured precisely as a result of this rotational imaging process. All keratometers and most corneal topographers do not see the central cornea, which gives the Pentacam a significant advantage over these other methods in terms of accuracy.
The “Holladay Report” feature of the Pentacam uses information from direct measurements of both the anterior and the posterior cornea within a 4.0 mm area to generate a central corneal power value in keratometric diopters. This value can then be used with the Holladay 2 formula for IOL power calculations, or with a 3^{rd} generation, 2-variable formula and an Aramberri double K method correction.
We have found the Pentacam to be one of the single most accurate measurement techniques to date for the eye with prior myopic LASIK.
Pentacam 4.0 mm Holladay Equivalent K value for the right eye: 40.30 D
Pentacam 4.0 mm Holladay Equivalent K value for the left eye: 41.00 D
Using this technique for estimating central corneal power, the following IOL powers are recommended by the Holladay 2 formula:
**SN60WF Target Refraction **
**Right** +17.50 D Plano
+18.00 D -0.38 D
**Left** +17.50 D Plano
+18.00 D -0.38 D
**Hard contact lens method **^{3, 6}
Following all forms of ablative keratorefractive surgery (LASIK, PRK, etc.) a review of the literature now suggests that the hard contact lens method may be less accurate than originally thought. For this reason it is no longer recommended in this clinical setting.
**Higher order optical aberrations following keratorefractive surgery**
The higher order optical aberrations that often accompany the various forms of keratorefractive surgery, such as an increase in positive spherical aberration (Z 4,0) and the multifocal nature of some of these corneas, will remain following cataract surgery. Understandably, some patients mistakenly expect that cataract surgery will alleviate these symptoms. Unfortunately, this is not the case. It is important to discuss this fact with these patients prior to surgery so that their expectations will be realistic. Also, the change from a prolate (steep central cornea) to an oblate (flattened central cornea) ocular system produced by lowering the central corneal power may result in decreased discrimination at higher spacial frequencies. This may not significantly improve after cataract surgery.
One helpful addition to our surgical armamentarium is the use of special lenses, such as the AMO Tecnis, or the Alcon IQ lens which help to reduce the addition of positive spherical aberration resulting from a flattened central cornea.
## Accuracy of intraocular lens power calculations following LASIK
It is important that our patients understand that intraocular lens power calculations following all forms of keratorefractive surgery are, at best, problematic and represent only an estimate. You should also discuss the fact that in spite of our very best efforts, the final refractive result may end up more hyperopic, or more myopic than expected. The fact that multiple methods are currently in use in this regard is eloquent testimony to how far we still have to go in developing a meaningful system of intraocular lens power calculations for the post-keratorefractive eye.
The possibility of an intraocular lens exchange, or a secondary piggyback implantation after all forms of refractive surgery, are important parts of informed consent prior to cataract surgery in this clinical setting. Given the limitations of available technology, this fact must be clearly understood by every patient as a well-recognized consequence of prior keratorefractive surgery.
**Regarding Monovision**
Many patients opt for monovision as part of their laser vision correction strategy for LASIK or PRK. This works well in the setting of laser vision correction, but for IOL power calculations following LASIK or PRK, the level of accuracy required to achieve this may not be possible. For monovision to work in a satisfactory manner, the IOL power calculation accuracy typically needs to be carried out to an accuracy of within **±**0.25 D, which is far beyond the resolution of this exercise. If your patient would like to be more myopic than the values obtained by normal calculation methods, you can do so by simply increasing the power of the IOL by approximately 1.4 times per diopter . However, the farther away from emmetropia, the less accurate the calculation becomes.
**IOL Power Determination**
Below is a summary of IOL powers, generated by several forms for central corneal power estimation. Some have certain characteristics, which we can use to better understand what the correct IOL power may be.
By bracketing between what is most likely an over-correction and what is most certainly an under-correction, it is possible to modestly improve the accuracy of an inherently inaccurate exercise. However, when refractive surgery results in a highly multifocal cornea, or there is unaccounted for lenticular myopia, this approach can show variable and unexpected results. When this system of bracketing breaks down, one or more pieces of the mathematical puzzle are either missing, masked, or inaccurate. Often, the calculations may be influenced our inability to determine the true post-LASIK refractive state (without the influence of lens-induced myopia). This is why for any of the historical methods that the post-LASIK refractive error is usually determined at four to six months after LASIK. This is long enough to be stable, but close enough to the procedure that lens-induced myopia does not become a factor.
It is generally accepted that IOL power calculations following keratorefractive surgery are typically placed on the myopic side of plano anywhere from -0.25 D to -0.75 D. This helps to lessen the possibility of unexpected post-operative hyperopia.
Below is the bracketing method for the patient whose calculations were carried out above.
**Calculation method IOL Power OD IOL Power OS**
Feiz and Mannis +17.50 D +16.50 D Sometimes will over-correct.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Clinical history +17.50 D +16.50 D
Modified Maloney +17.00 D +17.50 D
Masket method +17.50 D +17.00 D Correct IOL power is often in this
Modified Masket +18.00 D +17.50 D area between upper and lower limits
Corneal power adjustment +17.00 D +16.50 D
Corneal bypass method +18.00 D +19.00 D
Pentacam 4.0 HEK +17.50 D +18.00 D
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Measured Ks +16.00 D +15.50 D *Always *below correct IOL power
**Recommended power Right Eye Left Eye**
SN60WF (IQ lens) +17.50 D +17.50 D Best overall estimate of IOL power
Target refraction -0.38 D -0.38 D
**References**
1. Feiz V., Mannis M.J. Garcia-Ferrer F. Intraocular lens power calculation after laser in situ keratomileusis for myopia and hyperopia a standardized approach.
*Cornea* 2001; 20:792–797.
2. Koch, D., Wang I. Calculating IOL power in eyes that have had refractive surgery.
*J Cataract Refract Surg *2003 29(11) 2039-2042.
3. Holladay JT. Consultations in refractive surgery.
*Refract Corneal Surg *1989; 5:203
4. Hoffer KJ. Intraocular lens power calculation for eyes after refractive keratotomy.
*J Refract Surg* 1995; 11:490–493
5. Aramberri J. Intraocular lens power calculation after corneal refractive surgery:
Double K method. *J Cataract Refract Surg* 2003; 29: 2063–2068.
6. Haigis W. Corneal power after refractive surgery for myopia: contact lens method.
*J Cataract Refract Surg* 2003 29 (7) 1397-1411.
7. Seitz B. Intraocular lens power calculation in eyes after corneal refractive surgery.
*J Refract Surg* 2000; 16:349–361
8. Personal communication, April, 2004. Douglas D. Koch, MD, Cullen Eye Institute, Baylor College of Medicine, Houston, Texas.
9. Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculations methods in eyes that have undergone LASIK. Ophthalmology 2004 111(10) 1825-1831.
10. Masket S, Masket SE. Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser photoablation. *J Cataract Refract Surg* 2006 32 (3) 430 - 434.
11. Personal communication, April, 2006. Jack Holladay, MD, Houston, Texas.
12. Hill WE. IOL power calculations following keratorefractive surgery. Presented at Cornea Day of the Annual Meeting of the American Society of Cataract and Refractive Surgery, San Francisco, California, March 17, 2006
13. Holladay JT. Measuring corneal power after corneal refractive surgery. Insert to *Cataract and Refractive Surgery Today*. January, 2006. 4–6.
14. Walter KA, Gagnon MR, Hoopes PC, Dickenson PJ. Accurate intraocular lens power calculation after myoic laser in sitiu keratomileusis bypassing corneal power. J Cataract Refract Surg 2006 32 (3) 425 - 429. |