Factors affecting the forward shift of the posterior corneal surface in laser in situ keratomileusis

Abstract

Objective To investigate the forward shift of the posterior corneal surface and related influential factors after laser in situ keratomileusis (LASIK).Methods It was a prospective case series study.Eighty-two patients with myopia (38 males and 44 females) who had undergone LASIK at Union Hospital between December 2006 and February 2007 were retrospectively reviewed.The average age was 24.2±5.5 years,ranging from 18 to 44 years.No case of keratoconus was found during 3 years of follow-up.Intraocular pressure,corneal thickness and topography measurements were obtained with a Goldmann tonometer,ultrasonic corneal pachymeter and Orbscan Ⅱ z before and after surgery.All surgery was performed by the same surgeon using a 217Z100 Bausch & Lomb.Results After LASIK,the best fit sphere radius of the posterior corneal surface was shortened by 113.32±66.87 μm,with an average forward shift of 71.97±28.70 μm.The difference in the posterior corneal surface power between pre-and post-operation was 0.93±0.54 D.The forward shift of the posterior corneal surface showed a positive correlation with preoperative intraocular pressure (r=0.461,P＜0.01),and a negative correlation with residual stromal bed thickness (r=-0.453,P＜0.00).There was a forward shift of varying degrees in the posterior comeal surface in most cases,including several severe cases.But no corneal ectasia was observed during the follow-up period of 36 months.Conclusion The forward shift of the posterior corneal surface could be an inevitable response because of corneal thinning after LASIK,and a change in the posterior corneal surface showed a positive correlation with preoperative intraocular pressure and a negative correlation with residual stromal bed thickness. Key words: Intraocular pressure; Keratomileusis,laser in situ; Corneal topography; Forward shift of posterior corneal surface; Refractive regression