Changes in corneal thickness and refraction after LASIK

Objective To evaluate changes in corneal thickness and refraction after laser in situ keratomileusis(LASIK) for myopia,and the relevant factors that influence the results.Methods Seventy-six eyes of 52 patients with myopia or myopic astigmatism with spherical equivalent(SEQ) refractive errors of-2.75 to-12.50 diopters(D),[mean,(-6.71±2.35)D] were included in the study.Changes in refraction and changes in the thickness of the corneal apex were measured with scanning-slit corneal topography(Orbscan,Orbtek,Inc.) preoperatively and 1,2,and 6 months postoperatively.The mean forward thinning of the corneal apex and refraction before and after LASIK were compared and the changes were analyzed.Multiple regression analysis was used to assess the factors that affect the thinning of the corneal apex.Results After surgery,the posterior corneal surface displayed a mean forward shift(different-diff) of(38.81 ± 17.87) μm,(35.61 ±13.60)μm,and(36.45±14.34)μm,after 1 month,2 months and 6 months,respectively.The mean posterior corneal refraction(3.0 mm dia) was(-6.57±0.22)D preoperatively,and decreased to (-6.85±0.23)D,(-6.83±0.28)D and(-6.81±0.25)D,respectively,after 1 month, 2 months and 6 months.Mean posterior corneal refraction(5.0 mm dia) was(-6.26 ± 0.23)D preoperatively,and decreased to(-6.34 ±0.24)D,(-6.38±0.21)D and(6.39±0.25)D,respectively.The preoperative versus postoperative measurements were significantly different,but measurements taken at 1 month,2 months and 6 months postoperatively did not show statistically significant differences.There was a significant correlation between protrusion and refraction of the posterior corneal surface(r=0.6,P0.01).Variables that have an effect on the forward shift of the posterior corneal surface,in the order of the magnitude of the influence,are the amount of laser ablation(partial regression coefficient B=0.405,P0.01) and the thinnest corneas(B=-0.109,P0.01) preoperatively.Preoperative intraocular pressure correction showed collinearity with the amount of laser ablation and the thinnest corneas.Conclusion Laser in situ keratomileusis(LASIK) induced significant refractive changes in the posterior corneal surface and a forward shift of the cornea.Eyes with thinner corneas and higher myopia requiring greater laser ablation are more predisposed to an anterior shift of the cornea.