Time course of changes in corneal forward shift after excimer laser photorefractive keratectomy

Abstract

Background: Excimer laser refractive surgery has been reported to induce forward shift of the cornea, but its long-term sequelae remain unknown.

Objectives: To prospectively investigate the time course of changes in corneal elevation after excimer laser photorefractive keratectomy (PRK).

Methods: We performed PRK on 65 eyes of 34 patients with refractive errors of -1.25 to -10.0 diopters. The anterior/posterior corneal elevation and corneal thickness were measured with a scanning-slit corneal topography system before and 1 week and 1, 3, 6, and 12 months after surgery. Twenty eyes of 10 healthy control subjects underwent similar measurements at 3-month intervals.

Results: The posterior corneal surface displayed a mean +/- SD forward shift of 36.6 +/- 25.3 microm 1 week after PRK, which gradually increased to 55.1 +/- 46.1 microm at 1 year. All postoperative values were significantly larger than those of healthy controls (2.4 +/- 8.9 microm; P<.001, Mann-Whitney test). The largest forward shift occurred within the first postoperative week. The progression thereafter was most pronounced from 1 to 6 months, and nearly stabilized at 6 months. The variance of postoperative data was statistically significant (P<.001, repeated-measures analysis of variance). Multiple postoperative comparisons demonstrated significant differences between measurements at 1 week and 6 months (P =.002, Tukey Honestly Significant Difference), at 1 week and 1 year (P<.001), at 1 and 6 months (P<.001), and at 1 month and 1 year (P<.001). Progression of forward shift was more prominent in eyes with less preoperative corneal thickness and greater myopia that required larger laser ablation. We observed no progressive thinning and expansion of the cornea during the 1-year follow-up, which refuted the occurrence of true ectasia. A statistically significant correlation was found between the amount of myopic regression and the forward shift of the cornea (Pearson correlation coefficient, r = -0.37; P =.005).

Conclusions: Photorefreactive keratectomy induced forward shift of the cornea, which is not true corneal ectasia. The largest forward shift occurred within the first postoperative week. Changes were progressive up to 6 months postoperatively, but became almost stable thereafter. Eyes with thinner cornea and higher myopia, requiring greater photoablation, are more predisposed to progression. Forward shift of both corneal surfaces added to the tendency toward myopic regression after PRK.