Method for assessing the impact of residual roughness after corneal ablation in polychromatic vision

Abstract

Purpose : Despite theoretical models for achieving laser-based ablation smoothness, methods do not yet exist for assessing the impact of residual roughness after corneal ablation, on retinal polychromatic vision. We developed a method and performed an exploratory study to qualitatively and quantitatively analyze the impact of varying degree of corneal roughness on retinal image.

Methods : A preliminary version of the Indiana Retinal Image Simulator (J Opt SocAm A Opt Image SciVis. 2008 Oct;25(10):2395-407) was used to simulate the polychromatic retinal image. Using patient-specific Zernike coefficients and pupil diameter, the impact of different levels of chromatic aberrations was calculated. Corneal roughness was modeled via both random and filtered noise (Biomed. Opt. Express 4, 220-229 (2013)), using distinct pre-calculated higher order Zernike coefficient terms. The outcome measures for the simulation were simulated retinal image, Strehl Ratio and Visual Strehl Ratio in frequency domain (VSOTF). The impact of varying degree of roughness (with and without refractive error), spatial frequency of roughness, and pupil dilation was analyzed on the outcome measures. Standard simulation settings were pupil size = 6mm, Defocus = 2 μm (-1.54D), and Spherical Aberration = 0.15 μm. The 2^nd and 4^th Zernike orders formed the wavefront signal, while the 6^th and higher Zernike orders formed the noise signal. The 5^th Zernike order terms were set to zero to avoid overlapping of close Zernike orders.

Results : In case of a constant roughness term, reducing the pupil size resulted in improved outcome measures and simulated retinal image (VSOTF = 0.049 for pupil size = 6mm to VSOTF = 0.15 for pupil size = 3mm). The calculated image quality metrics deteriorated dramatically with increasing roughness (VSOTF = 0.163 standard setting; VSOTF = 0.05 for roughness 0.25µm; VSOTF = 0.0153 for roughness 0.6µm). Clear distinction was observed in outcome measures for corneal roughness simulated as random noise compared to filtered noise, further influenced by the spatial frequency of filtered noise.

Conclusions : The proposed method enables quantifying the impact of residual roughness in corneal ablation processes at relatively low cost and can help comparing different refractive laser platforms in terms of their associated roughness in ablation, indirectly improving the quality of results after Laser vision correction surgery.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.