Dr.Jai Gopal Agrawal,
Intraocular power calculation is challenging for patients who have previously undergone corneal refractive surgery.
Unlike nonoperated eyes, IOL power calculation after refractive surgery, first described by Holladay in 1989 and later by Koch et al,are problematic.
Although the sources of prediction errors for these eyes are well known, the solutions for eliminating them remain elusive even after using various methods of calculation.
There have been numerous formulas and calculation methods proposed for use in these eyes, and the process of employing them is typically time consuming and complex.
The aim of this study is to discuss some of the more popular and up-to-date solutions for IOL power calculation in eyes that have undergone myopic laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) surgery.
There are 3 main sources of errors in IOL power calculations for a patient that has undergone a myopic LASIK/PRK operation:
Radius errors: The frequency of their occurrence has been reduced with the introduction of newer forms of ablation that use a larger (6mm or more) effective optical zone and keratomretry/topography measure paracentral area which is well within ablated zone. As a result, these errors are now usually small (around 0.1 D).
Keratometer index errors: Keratometersand topographers measurements are based on anterior corneal curvature measurements and these assume a fixed ratio between the anterior and posterior surface of the cornea (keratometric index—usually 1.3375). It works well with virgin eyes.
But when the anterior corneal curvature has been altered by LASIK or PRK but the posterior curvature does not change, this ratio is disrupted, and the keratometric refractive index becomes invalid.This means that standard keratometry measurements should not be used in these eyes without adjustments to estimate the total corneal power.
This error can be addressed, with some degree of success,
-by various formulas that mathematically estimate the total corneal power
IOL formula errors. Standard IOL power formulae use the axial length and corneal power to predict the position of the IOL postoperatively. Current standard IOL formulae are the 3rd and 4th generation theoretic formulae such as the Hoffer Q, SRK/T and Holladay I.
The formulae use the corneal power in two ways.
First, corneal power is directly used in the vergence calculation to predict the postoperative refraction.
Second, corneal power is used in the prediction of effective lens position (ELP), which is the depth of the IOL relative to the cornea.
Refractive surgery changes the corneal power but not the depth of the lens, leading to an error in ELP prediction in the standard formulae. After myopic LVC, the corneal power is decreased and the ELP estimates become too low. This leads to an underestimation of the IOL power required and thus potentially a hyperopic prediction error in eyes that had previous myopic refractive surgery.
This issue was recognized by Aramberri, who described the ‘‘double-K’’ approach, in which the current(postrefractive) estimated total mean K is used for the vergence calculation and the prerefractive K is used to calculate the ELP.Out of the third and fourth generation formulae, Holladay 1, SRK/T and Hoffer Q, must follow this pattern of calculation.When no previous data are available the calculations are more misleading as no double-K adjustment can be done, unless an average value (eg, 43.13 D) is presumed to be the preoperative corneal power.
Haigis formula does not need double-K correction as it does not use K readings to calculate postoperative ELP.
Holladay 2 is integrated in the Holladay consulting program, so does not need double-K correction. If HAIGIS OR HOLLADAY 2 formula are used, this double –K correction is not necessary.
Purpose: To get correct IOL power after myopic LASIK/PRK and compare different powers obtained from ASCRS calculator and Aramberri adjustment for SRK/T.
Exclusion criteria
- Previous ocular trauma/surgery.
- Intra operative and post-operative complications.
- pre-existent ocular pathologies
- potential bag instability like patients with pseudoexfoliation syndrome
Patients:
Study done between Mar 2014 and April 2018
Number of patients—19 cataract pts
Age — 32-48yrs
Method:
History: All data regarding previous refractive surgery are taken from pts. It is mandatory to get data from documents issued by hospital only where refractive surgery was done.
- From five pts we get only prerefractive refraction glass power.
- From eleven pts we get no prerefractive data.
After thorough examination of the pts,
Corneal topography,
IOL master readings- K-readings, AL& Ant chamber depth W to W readings,
OCT(RTVue)- post corneal power, lens thickness & CCT were done.
We entered all the into ASCRS postrefractive IOL calculator.
We had used Masket&Savini + Double-K SRK/T for those (five pts) from whom we got prerefractive glass power.
For two cases I had used SRK/T formula correction table-Aramberri “Double K” method
For rest with no previous data we choose OCT, Barrett True K, Masket, Shammas &Haigis-L.
Results:
Predicted IOL power calculated for each. IOL prediction error was obtained by subtracting predicted IOL power from power of IOL implanted. Arithmetic IOL prediction errors, variances of mean arithmetic IOL prediction error, median refractive prediction error (RPE) and percent of eyes within 0.5 D and 1.0 D of RPE were calculated. Absolute RPEs were .57, .61, .47, .59, .60, .67, .51, .49 and .58 respectively for Masket, Modified Masket, Barrett True K, Shammas, Haigis-L, OCT, Barrett true K no history, Average and Aramberri adjustment (p < 0.15).
Conclusion: ASCRS calculator is promising. Formulas in ASCRS calculator OCT, Masket, Barrett True K, Shammas & Average give excellent results, but results from Aramberri adjustment using SRK/T are not very satisfactory.
Barrett True K gives maximum accuracy for predicted IOL power.
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