Impact of the corneal epithelium on the corneal power using 3D raytracing with OCT data.

Purpose: To study the effects of corneal imaging and focusing using a raytracing simulation with 2 and 3 surface corneal models based on customized surface representations of corneal tomography data .

Methods: Raytracing simulation using surface data for the epithelium (S1), stroma (S2) and endothelium (S3) extracted from MS-39 anterior segment tomographer CSV export files. Customized surface representations were derived using Gaussian Process Predictors, and rays traced through the cornea and a 3.5 mm aperture stop located 3.66 mm behind the corneal apex. 4 clinical examples were evaluated: A) after hyperopic LASIK, B) after myopic LASIK, C) keratoconus, and D) after PRK with postoperatively developed Salzmann nodules.

Results: The raytracing based bundle focus and wavefront focus distances of the 2 surface (S1 and S3) and 3 surface cornea models (S1, S2 and S3) were comparable, whereas the paraxial focus derived from a 1 surface cornea (S1), 2 (S1 and S3) or 3 surface cornea (S1, S2 and S3) using floating best fit sphere representations for S1, S1 and S3 showed systematically lower / higher focal distance with B) / C) indicating an overestimation / underestimation of corneal power with paraxial calculations.

Conclusions: The clinical examples in this study exhibited only minor differences between the mono- and dual layer cornea models. We recommend verification in a larger clinical study. Three surface corneal raytracing models could be of clinical relevance in intraocular lens calculations and LASIK ablation nomograms, offering potential improvements over paraxial calculations especially in cases with surface irregularities.