EXCIMER-LASER PHOTOREFRACTIVE KERATECTOMY FOR MYOPIA - COMPARISON OF 4.00-MILLIMETER AND 5.00-MILLIMETER ABLATION ZONES

BACKGROUND: To date, there has been no systematic study of the effects of ablation zone diameter on the outcome of photorefractive keratecto my. To address these issues, we examined a series of eyes with bilater al corrections using different-sized ablation zones. METHODS: Thirty-t hree patients underwent bilateral photorefractive keratectomy (Summit Excimed UV200, Waltham, Mass) with identical dioptric corrections in b oth eyes, except first eyes had 4.00-millimeter and second eyes had 5. 00-millimeter ablation zones. Identical postoperative eyedrop regimens were used in both eyes of each subject and the interval between treat ments was 12 months. The mean depth of the programmed central ablation was 24 mum in eyes treated with 4.00-millimeter and 39 mum with 5.00- millimeter zones. RESULTS: There was no statistically significant diff erence in the preoperative refraction between first and second eyes. M ean changes in refraction at 1, 3,6,9, and 12 months were significantl y greater in eyes treated with 5.00-millimeter ablation diameters (p < .001). No eyes treated with 4.00-millimeter zones were overcorrected, but five eyes (15%) treated with 5.00-millimeter beams had a refracti on greater than + 1.00 diopter (D) at 12 months postoperatively. There was no significant difference in the amount of anterior stromal haze between the two eyes at any stage. In 14 patents, less night halo was noticed in the eye treated with a 5.00-millimeter zone. Using a comput er program, halo measurements were made in both eyes of 12 patients wh ose pre- and postoperative refractions were within 0.50 D. The magnitu de of halo was significantly less in eyes treated with 5.00-millimeter zones (p < .01). CONCLUSIONS: Despite greater depths of stromal ablat ion with 5.00-millimeter diameters, there was no increased anterior st romal haze or postoperative regression of refraction. The biological a nd physical constraints governing the optimum size of the photorefract ive keratectomy ablation zone are discussed.