The Impact of Corneal Epithelium on Corneal Aberrations after LASIK

Abstract

Purpose: To quantify the contribution of the corneal epithelium to corneal power and higher order aberrations, especially spherical aberrations (SA) after myopic and hyperopic LASIK treatment.

Methods: The epithelium thickness profile was modeled for different magnitudes of spherical refractive error correction based on previous results (Reinstein et. al., 2009; Reinstein et. al., 2010). The traditional Munnerlyn equation was applied to simulate the postoperative corneal anterior surface profile. The posterior cornea surface with 6.8mm radius of curvature remained unchanged after ablation. The modeled epithelial thickness profiles were imported into commercially available optical design software (Zemax^TM Bellevue, WA) to analyze aberrations induced by the epithelium. This analysis was performed under two different corneal models, (1) single layer cornea i.e. the epithelium thickness was treated as corneal stroma and (2) two-layered cornea in which the refractive indices for the epithelium and corneal stroma were 1.433 and 1.376, respectively. The difference in the aberration profile between these two conditions represents the epithelium induced aberration. The aberrations analyzed included defocus, primary and higher order SAs (4^th, 6^th and 8^th Zernike radial orders).

Results: After myopic LASIK ranged from -1 to -13.5D correction, the epithelium thickness profile was thicker both centrally (7-13µm) and peripherally (2-9µm) compared to preoperative thickness. In contrast, hyperopic LASIK up to +8D correction caused central thinning (8-21µm) and peripheral thickening (10-72µm), demonstrating that variation in epithelium thickness across the cornea after hyperopic LASIK was increased substantially compared to myopic LASIK. In myopic LASIK, the epithelium layer caused little changes in defocus (< 0.05D) and SAs (< 0.01µm) which had almost no impact on overall optical quality of cornea. However, the epithelium layer after hyperopic LASIK induced 0.48D defocus on average, with a greater change with increasing refractive error correction (0.79D for a +8D correction). SAs also changed by 0.055um (4^th SA), 0.014um (6^th SA) and 0.013um (8^th SA).

Conclusions: Variation in the epithelium thickness after hyperopic refractive surgery influenced corneal aberration significantly. Therefore, epithelial healing response of the cornea is an important factor to consider for better postoperative refractive outcomes.