7 IOL Calculations after Refractive Surgery
7.1 Introduction
Although corneal refractive surgery produces excellent visual outcomes, it creates difficulties in accurately calculating intraocular lens (IOL) power. 1 This chapter discusses the problems induced by corneal refractive surgery including laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and radial keratotomy (RK), solutions to those problems, and currently available tools for IOL power calculations in these challenging cases, including Web-based IOL power calculation, an optical coherence tomography (OCT)-based IOL power formula, intraoperative measurements, and postoperative lens adjustments.
7.2 Problems Induced by Corneal Refractive Surgery
There are two major causes of error in IOL calculations in these eyes: incorrect corneal refractive power estimation obtained from standard keratometers or computerized videokeratography (CVK), and incorrect estimation of effective lens position (ELP) as calculated by most third- or fourth-generation IOL power calculation formulas.
Standard keratometry and simulated K-readings from CVK measure only four points or small zones on the anterior cornea in a paracentral region, ignoring the more central region altered by ablation. Furthermore, standard keratometry and CVK measure only the anterior corneal surface and assume the curvature of the posterior corneal surface. These devices use a standardized corneal refractive index, which is 1.3375 in most devices, to convert the anterior corneal measurement to an estimation of total corneal refractive power. (LASIK) PRK alter the ratio of anterior corneal curvature to posterior corneal curvature; therefore, the use of the standard corneal refractive index (1.3375) is no longer valid. Using the dual Scheimpflug analyzer (Galilei, Ziemer Ophthalmic Systems), we found that the mean corneal refractive indices were 1.3278 in normal eyes, 1.3246 in eyes with previous myopic LASIK/PRK, and 1.3302 in eyes with previous hyperopic LASIK/PRK. 2 Approaches for dealing with the errors in corneal refractive power estimation are discussed in detail in the next section.
Except for the Holladay II, Haigis, and Barrett formulas, third- and fourth-generation IOL formulas use measured corneal power to predict the effective lens position (ELP). The flattened K values after myopic LASIK/PRK will thus cause these formulas to predict a falsely shallow ELP and calculate insufficient IOL power, yielding a postoperative hyperopic surprise.
To solve this problem, Aramberri proposed the double-K method, 3 which uses two different K values: the preoperative K value for the estimation of ELP and the postoperative K value for calculation of IOL power in the standard vergence formula. This approach had been previously developed by Holladay in the Holladay 2 formula. Several studies have shown that the double-K method improves the accuracy of IOL power calculation after LASIK/PRK.s. Literatur , s. Literatur , 5 Nomograms proposed in our study can be used to adjust the IOL power when the modified corneal powers are used for these formulas. 6 With the Holladay 2 formula, one can enter the preoperative K value to predict the ELP. If previous data are not available, one can check the “previous us RK/PRK/LASIK” box, which will instruct the formula to use the default corneal power of 43.86 D for its ELP calculation.
In previous studies, we found that the magnitude of the ELP-related error varies according to the particular formula, the amount of refractive correction, and the axial length (AL) (Fig. 7.1).s. Literatur , 7 Comparing the SRK/T, Hoffer Q, Holladay 1, and Holladay 2 formulas, the single-K SRK/T formula produced the largest ELP-related prediction error, whereas the single-K Hoffer Q formula yielded the smallest. In 38 previously myopic and 24 previously hyperopic eyes that underwent phacoemulsification with IOL insertion after LASIK, Awwad et al 8 evaluated the effect of the double-K modification on third-generation formulas. They found that, among third-generation formulas, the double-K SRK/T overestimates IOL power in eyes with a large AL, especially with concomitant steep pre-LASIK keratometry. They also found the double-K Holladay 1 to be more accurate in postmyopic LASIK eyes.
7.3 Solutions to Problems Induced by Corneal Refractive Surgery
Several techniques have been proposed to improve the accuracy of IOL power calculation in eyes following corneal refractive surgery. These can be divided into four methods: (1) ignore current corneal measurements and use pre-LASIK/PRK data, (2) use current corneal measurements and modify them, (3) use current corneal measurements to calculate IOL power and modify the calculated IOL power, and (4) measure actual anterior and posterior corneal curvatures to calculate true corneal power. Table 7-1 provides an overview of the strengths and weaknesses of each approach.
Method | Strength | Weakness | Technique |
Ignore current corneal measurements and use pre-LASIK/PRK data |
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Use current corneal measurements and modify them |
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Use current corneal measurements to calculate IOL power and modify the IOL power |
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Measure actual anterior and posterior corneal curvatures to calculate true total corneal power |
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Abbreviations: ACCP, average central corneal power; TCP, total corneal power; ∆MR, change in manifest refraction; OCT, optical coherence tomography. | |||
7.3.1 Ignore Current Corneal Measurements and Use Pre-LASIK/PRK Data
These methods require the preoperative K readings and manifest refraction (MR), as well as the surgically induced change in MR (ΔMR). In these approaches, care should be taken to obtain accurate historical data, including the measurement of the post-LASIK stable refraction before the cataract developed.
The concern with this category is the dependence on the accuracy of preoperative data. There is at least a one-to-one diopter error if any historical data are incorrect. Historical data are often obtained from another office where one cannot verify the calibration of the keratometer. Also, it is often difficult to accurately determine when the post-LASIK refraction stabilized prior to cataract development. For all of these reasons, we have found that methods using historical data are less accurate compared with other methods.s. Literatur
Clinical History Method
With this method, postoperative corneal power is calculated by subtracting the ∆MR at the corneal plane induced by the refractive surgical procedure from the corneal power values obtained prior to refractive surgery.
Feiz–Mannis Method
This method 10 calculates the IOL power by using pre-LASIK/PRK keratometry values as if the patients had not undergone LASIK/PRK, and IOL power is calculated using pre-LASIK corneal power values and the axial length measured just prior to cataract surgery. The LASIK-induced change in refractive error divided by 0.7 is then added to this calculated IOL power to find the recommended IOL power.
Corneal Bypass Method
This method 11 uses pre-LASIK/PRK keratometry values, pre- and post-LASIK/PRK refractions, and the current AL measurement. It assumes that the patient had no LASIK/PRK procedure and calculates the IOL power by setting the target refraction to the pre-LASIK/PRK refraction or the net refractive correction, if the post-LASIK/PRK refraction is not plano.
7.3.2 Use Current Corneal Measurements and Modify Them
There are several methods that modify corneal power measurements based on regression analysis or on the amount of refractive change induced by the LASIK surgery. One advantage of these methods is that they use corneal data obtained at the time the patient presents for cataract surgery. They also avoid the one-to-one error found in approaches that rely entirely on historical data because they multiply the change in manifest refraction by some fraction, typically < 0.3.
Modification Based on Amount of Refractive Correction
Adjusted EyeSys EffRP
The (EffRP) from the EyeSys topographer (from EyeSys Vision, Inc.) is the mean corneal power over the central 3 mm zone, accounting for the Stiles–Crawford effect. Hamed et al proposed the adjusted EffRP, which is modified according to ΔMR as follows 12 , 13 :
Adjusted Atlas Values
Adjusted Atlas Ring Values
The average of the Atlas 0, 1, 2, and 3 mm annular rings (AnnCP) from the Atlas topographer (Carl Zeiss Meditec) is modified according to the ∆MR. With this method, we found that the accuracy of the IOL power calculation in eyes following hyperopic LASIK/PRK is significantly improved 13 :
Adjusted Atlas Zone Value
With this method, the 4 mm zone value obtained from the Atlas 9000 topographer is modified according to the amount of refractive correction induced by the surgery:
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