Analysis of the change in anterior and posterior corneal Q-values in myopic astigmatism patients before and after LASIK

Abstract

Objective To evaluate and compare the distribution of anterior and posterior corneal asphericity in young myopic astigmatism patients before and after laser in situ keratomileusis (LASIK) surgery, and to investigate the possible factors influencing change.Methods This was a prospective case-series study.Patients with myopic astigmatism (18～39 years old) were recruited from Beijing Tongren Eye Center.The Q-values of 20°,25°,30°,35°,40° of the anterior and posterior corneal surfaces (AQ and PQ) were registered using an ALLEGRO Oculyzer before and after LASIK surgery (1-3 months).Changes in AQ and PQ (△AQ and △PQ) were investigated.Correlations of AQ,PQ, △AQ and △PQ to the refractive index were also analyzed.Data were analyzed with a rank sum test and a Pearson correlation.Results Two hundred ninety-six eyes of 163 patients were studied.The AQ20,AQ25,AQ30,AQ35 and AQ40 of the cornea were -0.24±0.12,-0.28±0.12,-0.31±0.12,-0.35±0.12 and -0.33±0.27 before LASIK surgery,while the PQ20,PQ25,PQ30,PQ35 and PQ40 were 0.21±0.25,0.04±0.21,-0.10±0.19,-0.20±0.16 and -0.30±0.13.AQ values were negatively correlated with the cylindrical diopter (CD) (r=-0.213,-0.279,-0.332,-0.363,-0.133,P＜0.05).After LASIK surgery both the AQ and PQ values showed significantly changes,which were 0.96±0.55,0.94±0.48,0.80±0.40,0.51±0.35,0.25±0.36 for the AQs (U=38.15,43.94,48.47,44.78,25.97,P＜0.05) and 0.31±0.25,0.14±0.21,-0.02±0.19,-0.16±0.17,-0.29±0.15 for the pQs (U=9.10,10.18,9.77,7.75,3.30,P＜0.05).△AQ20,△AQ25,△AQ30,△AQ35 and △AQ40 showed positive correlation with spherical diopter (SD) (r=0.884,0.870,0.871,0.612,0.370,P＜0.05),spherical equivalent (SE) (r=0.834,0.866,0.872,0.625,0.388,P＜0.05) and the maximum ablation depth (MD) (r=0.779.0.823,0.842,0.634,0.403.P＜0.05).They were also negatively correlated with the residual depth of cornea stroma (CS) (r=-0.332,-0.377,-0.410,-0.352,-0.168,P＜0.05).△AQ35 and △AQ40 showed negative correlation with CD (r=0.124,0.120,P＜0.05).△PQ20,△PQ25 and △PQ30 showed negative correlation with SD (r=-0.210,-0.185,-0.154,P＜0.05),SE (r=-0.225,-0.196,-0.169,P＜0.05) and MD (r=-0.210,-0.184,-0.164,P＜0.05).△PQ35 were negatively correlated with SD (r=-0.121,P＜0.05)and SE (r=-0.124,P＜0.05).While △PQ40 only showed negative correlation with SD (r=-0.120,P＜0.05). Conclusion The anterior corneal surface of most young myopic astigmatic patients is prolate.The central part of posterior corneal surface is oblate,and the periphery part is prolate.They changes to oblate significantly after LASIK,especially the anterior corneal surface.Many factors affect the Q-values. Key words: Myopia; Keratomileusis,laser in situ; Corneal asphericity; Q-value