Short-term influence of LASIK on anterior and posterior corneal asphericity

Abstract

Background Laser in situ keratomileusis (LASIK) is frequently performed to reduce or eliminate myopic refractive error.Some patients complain of a loss of visual function after surgery even when they have 20/20 visual acuity.One of the reasons is the change of asphericity of the cornea.Objective This study attempts to investigate the short-term changes of the anterior and posterior corneal asphericity after LASIK.Methods One hundred and seven myopic eyes of 54 subjects with the equivalent spherical diopter of -1.30 to -7.50 D who have received LASIK were enrolled in this prospective study.The Q-values of the posterior corneal surface for different corneal diameters (6mm,7mm,8mm and 9mm) were measured with Pentacam preoperatively and 1 month postoperatively.The correlations between Q-value,Q change (△Q),and the mean preoperative spherical equivalent refraction (SE),central corneal thickness (CCT),central ablation depth (AD) and residual bed thickness were investigated.Written informed consents were obtained from all the subjects prior to the clinical trial.Results The Q-values of the anterior and posterior corneal surfaces gradually decreased to negative values with the increase of corneal diameter in myopic eyes.Weak correlations were found between the asphericity of the anterior and posterior corneal surfaces from diameters of 6mm,7mm,8mm and 9mm (r=0.227,0.288,0.303,0.389;P=0.019,0.003,0.002 and 0.000,respectively).No statistically significant correlation was found between Q-value and the diopter of refractive error (P＞0.05).Both the anterior and posterior corneal Q-values varied toward the positive direction except that in the 9mm area of the posterior corneal surface at postoperative 1 month(t=-1.495,P=0.138).The increase of the anterior corneal asphericity (△Q) was more obvious than that of the posterior corneal surface and showed a positive correlation with ablation depth and a negative correlation with residual bed thickness.However,no statistically significant correlation was seen between △Q and these two parameters in the posterior corneal surface (P＞0.05).Conclusion The shape of the anterior and posterior corneal surface shows more prolateness as the increase of corneal diameter in myopic eyes.Both the anterior and the posterior corneal surfaces have an oblate shift within the ablation zone 1 month after LASIK. Key words: myopia; excimer laser; asphericity; Q-value