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J. Einighammer, T. Oltrup, T. Bende, B. Jean; Simulation versus Real Outcome of Corneal Ablations Investigated by Real Ray Tracing. Invest. Ophthalmol. Vis. Sci. 2009;50(13):576.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the recently reported discrepancy between theoretical expectations and clinically found outcome of corneal ablations during refractive laser surgery. Based on measured topographies, corneal shape and wavefront error (WFE) calculated by real ray tracing (RRT) are compared.
34 eyes of 25 patients who had LASEK with a flying-spot excimer laser (ESIRIS, SCHWIND eye-tech-solutions GmbH, Germany) for myopia correction (-6.26±2.52 D, ablation diameter 6.25-7.00 mm) without any astigmatism correction were assessed. Topographies were measured with a C-Scan (Technomed GmbH, Germany) topography system preoperatively (pre-op) and postoperatively (post-op). Based on the pre-op topographies, corneal ablation is simulated using Munnerlyn’s formula. To include the fluence loss of the laser, one simulation is done with homogeneous beam fluence (HBF) and another with variable beam fluence (VBF, including reflection loss and non-normal laser beam incidence). From the preo-op, simulated and measured post-op topographies the corneal shape in terms of apical radius and asphericity is determined with a biconic surface fit. Furthermore the corneal WFE is calculated with RRT, fitted to Zernike coefficients and reported as higher order aberrations (HOAs, 3rd to 6th) and spherical aberration Z(4,0) according to the OSA standard.
See table for detailed results. The pre-op and post-op simulated corneas are prolate, while the post-op measured corneas are clearly oblate. HOAs and Z(4,0) are not increased for the post-op simulation, in contrast both are clearly increased for the post-op measurements.
The simulations with RRT show that the increase of oblateness, HOAs and especially Z(4,0) is not due to the Munnerlyn’s ablation profile itself. The fluence loss of the laser has been identified to be one significant factor. However, the simulations with RRT suggest that there are other more important factors left which have great influence on the post-op corneal shape, probably wound healing and corneal biomechanics.
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