Causes of increased corneal aspherical index following LASIK for myopia / 中华实验眼科杂志

ABSTRACT

Background Laser assisted in situ keratomileusis (LASIK) for myopia will change corneal asphericity and further affect the quality of vision.However,how operating parameters such as ablation depth and optic zone diameter affect corneal asphericity is still rarely reported.Objective Aim of this study was to investigate the influence of corneal ablation depth and optic zone diameter on corneal asphericity after LASIK for myopia.Methods This prospective study comprised 175 eyes of 89 patients with the spherical equivalent of (-5.93± 1.98)D and the best corrected visual acuity (BCVA) ≥ 1.0.The flap creation with the femtosecond laser combine iris recognize guided LASIK was performed on the patients who were available for the evaluation at postoperative 6 months.Corneal aspherical index (Q value) at the central corneal 6.0 mm were measured with Orbscan IIz-corneal topography before and 6 months after operation.Corneal ablation depth and optic zone diameter were recorded in operation.The changes of visual acuity and spherical equivalent before and after surgery were compared by self-control method,and the correlations between corneal ablation depth or optic zone diameter with alteration of Q value (AQ) after LASIK were assessed using multiple regression analysis.Results The mean spherical power,cylinder diopter and spherical equivalent were (-5.57 ± 1.89) D,(-0.71 ±0.55) D and (-5.93 ± 1.98) D before surgery,and those after surgery were (-0.25 ±0.30),(-0.14±0.22)D and (-0.32±0.37)D,showing significant differences between before and 6 months after LASIK (t=-32.39,-23.91,-35.18,all at P＜0.01).The Q values at the central corneal 6.0 mm were -0.13 ± 0.09 (-0.47-0.08) in preoperation and 1.09 ± 0.54 (0.22-2.51) in postoperation,with a significant increase in postoperation (t=29.37,P＜0.01).Corneal ablation depth was (95±28) μm and optic zone diameter was (6.32±0.26)mm.Ablation depth appeared to be positive correlation with AQ (β =0.803),and optic zone diameter showed a negative correlation with AQ (β =-0.149),with a multiple regression formula AQ =1.517+0.015×ablation depth-0.3 ×optic zone diameter.Conclusions LASIK for myopia increase corneal Q value.The increase of corneal ablation depth and decrease of optic zone diameter contribute to enlargement of corneal Q value.