CORNEAL ABLATION PROFILOMETRY AND STEEP CENTRAL ISLANDS

BACKGROUND: Photorefractive keratectomy with large diameter ablations using a uniform laser beam has produced central undercorrections, or ' 'steep central islands'' in patients, as seen with videokeratography. METHODS: Using a custom optical profilometer to measure corneal ablati on profiles and a VISX excimer laser system, we measured the effect of ablation algorithms, diameter, depth, and dioptric correction on enuc leated porcine eyes and living rabbit eyes. Our profilometer was verif ied using a 43.00 diopter (D) spherical surface and a 35.00 and 43.00 D bicurve test surface as a model for the ablated cornea. RESULTS: The profilometer measured the test surfaces to within 3 mu m of predicted values. Photorefractive keratectomies showed over-ablation peripheral ly and under-ablation centrally which increased with ablation diameter and dioptric correction. Fixed diameter ablations 2 to 6 mm in diamet er and 10 to 80 mu m deep showed stromal ablation rates vary spatially but not with ablation depth. These spatially variant ablation profile s were used to re-engineer the ablation algorithm and to produce photo refractive keratectomies with improved sphericity. CONCLUSIONS: Steep central islands are caused by the spatial variance of tissue ablated w ith a uniform laser beam irradiance. This aberration can be corrected by modifying the laser ablation algorithm to correct for the spatial v ariance of stromal ablation.