Ray Tracing Iol Calculation After Hyperopic Keratorefractive Lenticule Extraction: Impact Of Pupil Size And Corneal Asphericity

To identify the accuracy limits of ray-tracing–based intraocular lens (IOL) power calculation after hyperopia-correcting keratorefractive lenticule extraction (KLEx) and to determine which postoperative corneal and optical parameters drive prediction error, with special attention to pupil-size–dependent best-focus shifts.

Single-center retrospective analysis of the Marburg cohort from a prospective multicenter clinical approval study for hyperopic small incision lenticule extraction (SMILE).

Pre- and postoperative corneal tomography (Pentacam) and subjective refraction were available for 36 eyes (18 patients) treated with KLEx. Tomographies were imported into OKULIX ray-tracing software. IOL residual refraction was calculated pre- and post-KLEx using identical assumptions (axial length 23.6 mm, pseudophakic anterior chamber depth 4.0 mm, fixed IOL power 21.0 diopters (D)). Two IOL models with different spherical-aberration design were simulated (AR40e and Tecnis ZCB00). Prediction error was defined as the difference between (I) the change in calculated residual refraction (post–pre) and (II) the measured refractive change. These analyses were repeated for different pupil diameters.

KLEx produced highly prolate corneas with markedly increased (negative) spherical aberration, leading to pronounced pupil-size–dependent best-focus shifts. Depending on pupil diameter, the calculated refractive change differed by up to about 3.0 D in highly corrected eyes. The mean prediction error became zero at a pupil diameter close to 3.0 mm for both IOL models (3.01 mm AR40e; 3.05 mm Tecnis ZCB00). At these diameters, prediction-error standard deviation was 0.65 D and 0.66 D; mean absolute error 0.48 D for both; median absolute error 0.39 D and 0.40 D (AR40e and Tecnis ZCB00, respectively).

After hyperopic KLEx, IOL power prediction is highly limited by postoperative corneal asphericity and the resulting pupil-dependent focus shift from high spherical aberration. Full-aperture ray tracing with a clinically chosen target pupil diameter is therefore required since Gaussian optics–based formulas cannot model this effect. Aberration-correcting aspheric IOLs should be avoided because they may further increase negative spherical aberration in these eyes. Patients should be counselled regarding illumination-related refractive changes and depth-of-focus effects.