Topography and Raytracing Analysis of Patients With Excellent Visual Acuity 3 Months After Excimer Laser Photorefractive Keratectomy for Myopia

We performed topography and raytracing analysis 3 months after surgery on five consecutive eyes of five patients, which had excimer laser photorefractive keratectomy for myopia. Three of the five eyes had uncorrected postoperative visual acuity of 20/20 or better. Two eyes had an uncorrected vision of 20/40. In three of five eyes, the area of excimer ablation was centered within 1.0 mm of the optical axis. Three other eyes showed decentration that ranged from 1.1 to 1.5 mm. The range of surface power seen within 2 mm of the central keratoscope ring was as follows: patient CK = 37.50 to 39.50 diopters; patient CA = 40.50 D to 44.80 D; patient CW = 37.90 D to 42.20 D; patient AC = 35.50 D to 39.00 D; patient DT = 34.50 D to 41 .40 D. Topography patterns differed from eye to eye. A raytracing program modeled refraction of 20/80 and 20/20 "E" of 100%, 50%, 25%, 12.5% and 6.25% contrast through all measured points on the central 10 keratoscope rings of the five postoperative corneas. The five computer-derived images were ranked subjectively according to the observed degree of image degradation by three observers. Two eyes showed discernible 20/20 E's even at the 1 2.5% contrast level. Little to no ghost image was seen. Two eyes showed degraded but discernible 20/20 letters at higher levels of contrast only. These eyes showed moderate ghost images that were most apparent in the high-contrast 20/ 80 letters. One eye showed poor resolution of the 100% contrast 20/20 letter and moderately severe ghost images. The amount of image degradation correlated with the percentage of the total point spread function that fell within a 9 ? 9 pixel patch around the visual axis in the image plane of best focus (r = -1.0O1P= .01). These results suggest that excellent uncorrected Snellen visual acuity may occur in patients after excimer laser photorefractive keratectomy for myopia even when the postoperative surface shows a moderate amount of irregular astigmatism and moderate decentration of the area of ablation relative to the optical axis. Raytracing analysis suggests that differences in postoperative topography lead to differences in image quality that can be reproducibly ranked by independent observers. (Refract Corneal Surg 1991 ;7: 122- 128.)