Real Ray Tracing Simulation Versus Clinical Outcomes of Corneal Excimer Laser Surface Ablations

Abstract

To investigate the recently reported discrepancy between theoretical expectations and clinical outcomes of corneal ablations after excimer laser corneal refractive surgery. Thirty-four eyes of 25 patients who had laser epithelial keratomileusis (LASEK) for myopia (mean -6.26±2.52 diopters [D], full correction diameter 6.71±0.25 mm) without astigmatism correction were assessed. Based on preoperative topographies, corneal ablation was simulated using the Munnerlyn formula--one with homogeneous beam fluence and another with variable beam fluence (including reflection loss and non-normal laser beam incidence). Corneal shape was analyzed pre- and postoperatively. Corneal wavefront aberration was calculated with real ray tracing and reported according to the Optical Society of America standard. Corneal asphericity for a 6.0-mm diameter showed that preoperative measured corneas (-0.21±0.11) and postoperative simulated corneas (homogeneous beam fluence -0.32±0.19, variable beam fluence -0.41±0.22) were prolate, whereas postoperative measured corneas (0.40±0.57) were oblate. Corneal wavefront aberration (higher order aberrations 3rd to 6th/spherical aberration Z[4,0] in microns) for a 6.0-mm diameter, compared to the preoperative state (0.34±0.19/0.15±0.09), did not increase in postoperative simulation (homogeneous beam fluence 0.29±0.15/0.05±0.08, variable beam fluence 0.34±0.14/0.16±0.08), but in contrast increased for postoperative measurements (0.64±0.17/0.49±0.15). The increase in oblateness, higher order aberrations, and spherical aberration of real patients' postoperative corneas was not seen in simulations using a Munnerlyn ablation profile. The fluence loss of the laser was one important factor, but did not explain all increasing aberrations observed clinically. We hypothesize that corneal wound healing and biomechanics play a role.