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 ablation profiles and a VISX excimer laser system, we measured the effect of ablation algorithms, diameter, depth, and dioptric correction on enucleated porcine eyes and living rabbit eyes. Our profilometer was verified 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 microns of predicted values. Photorefractive keratectomies showed over-ablation peripherally and under-ablation centrally which increased with ablation diameter and dioptric correction. Fixed diameter ablations 2 to 6 mm in diameter and 10 to 80 microns deep showed stromal ablation rates vary spatially but not with ablation depth. These spatially variant ablation profiles were used to re-engineer the ablation algorithm and to produce photorefractive keratectomies with improved sphericity. CONCLUSIONS: Steep central islands are caused by the spatial variance of tissue ablated with a uniform laser beam irradiance. This aberration can be corrected by modifying the laser ablation algorithm to correct for the spatial variance of stromal ablation.

Treatment of myopic astigmatism with the 193 nm excimer laser utilizing aperture elements.

In this paper, we examine the 193 nm excimer laser's efficacy and safety in treating myopic astigmatism. The VISX Twenty/Twenty excimer laser uses aperture elements to effect astigmatic photorefractive keratectomy. In 70 patients with six months follow-up, the average postoperative sphere was -0.14 and the average postoperative cylinder was -0.54. Seventy-one percent of these patients had uncorrected visual acuity of 20/40 or better at six months. In 12 patients with one year follow-up, the average postoperative sphere was -0.05 and the average postoperative cylinder was -0.59. Eighty-three percent of these patients had uncorrected acuities of 20/40 or better; none had a clinically significant loss of best corrected acuity. This investigation demonstrates that the excimer laser can be used to treat myopic astigmatism successfully.