IOL Intraocular Lens Power Calculations Following LASIK and PRK
LASIK and PRK IOL Intraocular Lens Power Calculations After Post Eye Cataract Surgery Eyes
2008-09-13
|
Please note: the DOWNLOAD COPY (Microsoft Word file) aligns itself better on each page. The version below is provided for those who prefer not to download a copy. |
|
IOL Intraocular Lens Calculations 
|
|
Determining Corneal Power Following Myopic LASIK. |
|
The following information contains an outline of several popular central corneal
power calculation methods that can be used after the various ablative forms
of keratorefractive surgery for myopia, such as LASIK and PRK, for IOL power
calculations. As a general rule, IOL power calculations following all forms of keratorefractive surgery should not be run using an uncorrected 2-variable, third generation formula. Instead, the "double K" feature of the Holladay 2 formula (the small box labeled "Prior RK, LASIK...") should be employed in this setting. If your office does not have the Holladay IOL Consultant software package, a trial version can be downloaded from the Internet at:
Most 2-variable formulas such as SRK/T, assume that the anterior and posterior segments of the eye are more or less proportional and use axial length, and keratometric corneal power, to estimate the postoperative location of the IOL, known as the effective lens position (ELPo). Unless a special correction is made, the artifact of flat Ks following keratorefractive surgery will cause these formulas assume a falsely shallow post-operative ELPo.5 The end result is that 2-variable formulas following RK, PRK and LASIK will typically recommend less IOL power than required. This is a second and little recognized source of unanticipated post-operative hyperopia. This potential pitfall can be avoided by using the double K feature of the Holladay 2 formula, or applying Aramberri's "double K method" correction to the Holladay 1, Hoffer Q or SRK/T formulas. (see references) For the sake of illustration, we will run through the corneal power estimation and IOL power calculation for an actual patient from our office.
Clinical history method 3,4 It is important to have accurate pre-LASIK refractive data for this calculation. The clinical history method for corneal power estimation was first described by Holladay, and later by Hoffer as:
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 manual keratometers or corneal topographers (1.3333 vs. 1.3375). 7 Corrected for an estimated vertex distance of 13 mm, the historical estimation of the central corneal power of our patient for the right eye following LASIK would be carried out as follows:
Feiz and Mannis IOL power adjustment method 1 Another method that is helpful to use when good historical data is available is the IOL power adjustment method of Feiz and Mannis. With a target refraction set for -0.50D, this is the method least likely to result in a post-operative hyperopic surprise. As you will see at the end of this exercise, we will be able to use this method to set an upper limit of possible IOL powers. Using this technique, first the IOL power is calculated using the pre-LASIK corneal power as though the patient had not undergone keratorefractive surgery. This pre-LASIK IOL power is then increased by the amount of refractive change at the spectacle plane divided by 0.7. The calculation is for a -0.50 D result. This approach is outlined as follows:
The Feiz and Mannis IOL power adjustment method for the right eye following LASIK would be carried out as follows:
Modified Maloney method 2,9 Another very useful 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 a very low variance when used with either the Holliday 2 formula or the 2-variable formula "double K method" correction nomogram published by Koch and Wang.2 Using this technique, the central corneal power is obtained 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. |
|
(CCP x 1.114) - 6.1 D = Post-LASIK adjusted corneal power
Where...
CCP = the corneal power with the cursor in the center of the topographic map. |
The Modified Maloney method for the right eye following LASIK is carried out as follows:
Postoperative regression method 8 Yet another method of post-LASIK corneal power estimation is the postoperative regression method that has been shared with us by Doug Koch and Li Wang. Using this technique, the 0 mm, 1 mm and 2 mm annular rings of the Numerical View of the Zeiss Humphrey Atlas topographer are averaged together. This topographic-based averaged central corneal power is then multiplied by 1.23 and then 10.41 is then subtracted from this product. Like the Modified Maloney method, the advantage of this method is that it requires no historical data.
The postoperative regression method for the right eye following LASIK would be carried out as follows:
Topographic central corneal power adjustment method 2,9 The last 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 (EffRPadj) of the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System. Using this technique, the 1 mm and 2 mm annular power rings of the Numerical View of the Zeiss Humphrey Atlas topographer are averaged together, or the adjusted effective refractive power (EffRPadj) is determined, and that figure is reduced by 19% for every diopter of myopia corrected by LASIK. |
|
|
The topographic central corneal power adjustment method for the right eye would be:
Hard contact lens method 3,6 Following all forms of ablative keratorefractive surgery (LASIK, PRK, etc.) the literature now suggests that the hard contact lens method may be less accurate than originally thought. For this reason it is not 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, and the multifocal nature of some of these corneas, will remain unchanged 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. For example, the third and fourth order higher order aberrations produced by radial keratotomy can be as much as 35 times normal values. Significantly elevated third and fourth order aberrations are also seen following some of the early forms of LASIK that employed a large spot size and a non-Gaussian distribution strategy. 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 will not change after cataract surgery. Accuracy of intraocular lens power calculations following LASIK It is important to explain to your patient in no uncertain terms that intraocular lens power calculations following all forms of keratorefractive surgery are, at best, problematic. 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 five different methods are in use is eloquent testimony to how far we still have to go in this area. It is a disappointment to all of us that given available technology, there is simply no single reliable method to accurately determine the net central power of these unusual eyes. Intraocular lens exchange or secondary piggyback implantation after all forms of refractive surgery, are important parts of informed consent prior to cataract surgery. Given the limitations of available technology, this fact must be clearly understood by every patient as a well-recognized consequence of prior keratorefractive surgery. 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 effectively bracketing, 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 system 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). It is generally accepted that IOL power calculations following keratorefractive surgery are always placed on the myopic side and are typically for -0.50 D. This helps to prevent unexpected post-operative hyperopia.
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. |