Refractive Coupling Effects in Keratorefractive Lenticule Extraction Procedures: A Machine Learning-Assisted Approach

Abstract

Purpose: To evaluate the impact of refractive couplings in myopia and myopic astigmatism with two different keratorefractive lenticule extraction (KLEx) systems.

Methods: This was a retrospective evaluation of refractive outcomes with two different lasers studying 2,841 eyes undergoing small incision lenticule extraction (SMILE) (VisuMax 500; Carl Zeiss Meditec) and 2,528 eyes undergoing SmartSight (ATOS; SCHWIND eye-tech-solutions GmbH). Coupling effects (derived from the ratio between partial slopes) were determined for sphere and cylinder and for spherical equivalent, cardinal, and oblique astigmatism separately.

Results: Statistically significant coupling effects were observed for both the VisuMax and ATOS systems, as indicated by P values less than .05. For the VisuMax, a coupling effect of 8% of cylinder into sphere and a 2% coupling of sphere into cylinder was found. For the ATOS, the coupling effect of sphere into cylinder was 1%. A 3% coupling effect of oblique astigmatism into cardinal astigmatism in the VisuMax, and conversely, a 0.1% coupling effect of defocus into oblique astigmatism in the ATOS were found. In cases with no astigmatism plan, sphere had a 2% effect on induced astigmatism in the VisuMax and 0.6% in the ATOS. In high astigmatism plans, sphere had a significant 16% impact on cylinder in the VisuMax. Additionally, the effect of defocus on cardinal astigmatism was 6% in the VisuMax and 0.8% on oblique astigmatism in the ATOS.

Conclusions: Despite the P values less than .05 indicating statistical significance, the observed coupling effects were consistently low, with magnitudes below 10%, even for astigmatism exceeding 2.50 diopters. These couplings may be partly attributed to cross-effects of torsional eye movements. Coupling values for KLEx were markedly lower than those reported for non-aspheric excimer laser ablations. The results suggest that surgical results may be refined further by optimizing nomograms to mitigate coupling effects. [J Refract Surg. 2024;40(10):e706-e715.].