Postsurgery corneal asphericity and spherical aberration due to ablation efficiency reduction and corneal remodelling in refractive surgeries

Abstract

To explain the origin of changes in corneal asphericity and induced spherical aberration after laser refractive surgery. A rigorous model, CASIM (corneal ablation simulator), has been developed to model the ablation profile design, the shot-by-shot ablation process and the corneal remodelling that occurs through healing. The dependence of corneal asphericity, induced spherical aberration, and achieved refractive correction on corneal remodelling, and the ablation efficiency reduction caused by the angle of incidence of the excimer beam on the curved cornea are compared to the clinical outcomes reported in the literature. When the exact Munnerlyn formula is used, the CASIM modelling and the clinical data exhibit a high degree of correlation. The modelling predicts that the postoperative cornea will be oblate, with substantial induced spherical aberration. A 6-month postsurgery asphericity is predicted by CASIM with a correlation of R2=0.94. The corneal remodelling included in CASIM accounts, on the average, for 45 and 69% of the increase in asphericity and spherical aberration, respectively, with the remainder due to the ablation efficiency. The modelling shows that clinically observed increases in corneal asphericity and induction of spherical aberration can be explained by the effects of corneal remodelling due to healing and by the ablation efficiency reduction due to laser angle of incidence. The model is capable of predicting clinical outcomes for procedures performed with flying spot laser systems and could be used to design compensated ablation profiles to improve the clinical outcomes for custom as well as conventional laser refractive procedures.