Time Course of Changes in Corneal Forward Shift after Laser in situ Keratomileusis

Abstract

Abstract: : Purpose: It has been known that excimer laser keratorefractive surgery induces forward shift of the cornea. One-year longitudinal study indicated that corneal forward shift was progressive up to 6 months after photorefractive keratectomy, but such study has not been reported after laser in situ keratomileusis (LASIK). The current study was conducted to prospectively investigate the time course of changes in corneal elevation after LASIK. Methods: We performed LASIK on 93 eyes of 49 patients with myopia or myopic astigmatism (male 20, female 29). Their age was 35.1 ± 10.1 years old (mean ± SD) and the amount of correction was 6.5 ± 2.9 D. LASIK was performed with the excimer laser system VISX STAR S2 and the microkeratome Nidek MK-2000. The optical zone was 6.0 mm in diameter and the transition zone was set to be 0.35 mm. Corneal topography of the posterior corneal surface was obtained with the scanning-slit topography system (Orbscan II, Bausch & Lomb) before and 1 week, 1, 3, 6, and 12 months after surgery. Changes in the elevation of the posterior corneal surface were evaluated at the center of the difference map generated from preoperative and postoperative elevation maps. For surface alignment in the difference map, the 3-mm wide peripheral annular fit-zone was used. Results: Forward shift of the posterior corneal surface relative to the preoperative elevation was 54.7 ± 23.9 µm at 1 week, 56.8 ± 28.4 µm at 1 month, 53.9 ± 26.0 µm at 3 month, 47.6 ± 23.4 µm at 6 month, 50.0 ± 24.0 µm at 1 year. There was no significant fluctuation among the values obtained at each measurement point (p=0.108, repeated-measure ANOVA).The stepwise multiple regression analysis revealed that the factors relevant to the amount of the forward shift of the posterior corneal surface were the amount of ablation (B=1.806, p<0.001), magnitude of myopic correction (B=12.453, p=0.002), and preoperative corneal thickness (B=-0.197, p=0.024). Conclusions: The forward shift of the cornea is not progressive after LASIK and is different from true corneal ectasia.

Keywords: refractive surgery: LASIK • refractive surgery: corneal topography