Modeling Corneal Biomechanical Response To Laser Refractive Surgeries: Does The Mechanics Affect The Refractive Outcome?

Nowadays, laser refractive surgery represents clinical practice for permanent correction of medium-low vision’s defects. By means of a laser, corneal surface is reshaped to change its curvature and, consequently, its refractive power in order to achieve the desired dioptric correction. In this work, we present a comparison of two different refractive procedures, Photorefractive Keratectomy (PRK) and Small Incision Lenticule Extraction (SMILE), by creating digital twins with the numerical modeling technique: we analyzed the influence of the mechanical response of the corneal tissue on the final refractive outcome to the different refractive procedures.

In order to build the corneal models, clinical data (n=10 for each digital twin) in terms of ablation profile and central ablation depth were retrieved from the laser machines used in a tertiary referral hospital (Centro de Oftalmología Barraquer, Spain), the excimer laser Wavelight EX500 by Alcon for PRK and the femtosecond laser Visumax by Zeiss for SMILE, and compared to theoretical ablation profiles, found in literature (Munnerlyn), which are lower with respect to the clinical values. 

20 average finite element corneal models were built by considering 5 dioptric targets, ranging from -1 D to -5 D of myopic correction, for both PRK and SMILE, by applying theoretical and clinical ablation depths. Moreover, in SMILE surgery a 10% is added to the target correction, to achieve the refractive target, and 15 μm are added to the whole ablation profile to avoid lenticule rupture when it is extracted. The surgery simulation was performed in two steps: first, a physiological intraocular pressure (IOP) of 15 mmHg is applied to the corneal posterior surface; then, the laser surgery is performed. The total corneal refractive power was computed to analyze the refractive outcomes of the simulations.

For low dioptric corrections (up to -3 D) in PRK, both theoretical and clinical ablations reached the target correction, while an undercorrection of 0.5 D with the theoretical ablation was obtained for higher diopters. In these cases, the corneal mechanical response to the surgery was higher since a thicker ablation was removed and, thus, cornea deformed more. In SMILE a poor optical result was obtained with the theoretical ablation depth (undercorrection of 2 D). When applying the clinical ablation depth, we reached an accurate result, close to the target. In SMILE corneal model deformed more with respect to PRK and stresses concentrated in the volume under the lenticule, due to the discontinuity introduced by tissue’s removal. 

For PRK simulations the effect of the corneal biomechanical deformation is visible in high diopters, causing a low undercorrection with respect to the target. Lenticule extraction in SMILE surgery causes a higher tissue mechanical response and the difference between theoretical and clinical ablations is clinically relevant. Moreover, in SMILE surgery a thicker ablation profile is removed with respect to PRK to be able to achieve the desired correction, due to higher deformations induced by the surgery. Consequently, a thinner residual stromal bed is left with respect to PRK at the same refractive target. This study will be performed using patient-specific data, to analyze the impact of refractive surgeries on corneal biomechanics.