Decentration of photorefractive keratectomy ablation zones after excimer laser surgery for myopia

Purpose: To devise a method for measuring excimer laser photorefractive keratectomy (PRK) ablation zone decentration. Setting: Excimer Laser Clinic, Sunderland Eye Infirmary, Sunderland Tyne and Wear, England. Methods: Fifty-three eyes of 53 consecutive patients having PRK for a spherical myopic refractive error were studied. Preoperative and postoperative corneal topographic maps and differential subtraction maps were produced for each cornea. Using these maps, the distance between ablation zone centers and pupil centers was measured. All measurements were repeated by a second observer. Results: Mean ablation zone decentration was 0.46 mm from the pupil center; 69% of cases decentered 0.50 mm or less. The difference between the two independent measurements was 0.05 ± 0.06 mm (mean ± SD). A trend toward superonasal displacement was noted. Conclusion: This simple method of measuring zone decentration can be used to compare the refractive results of PRK with those of other procedures.