Changes Of Posterior Corneal Elevation And High Order Aberration After Femtosecond Laser-assisted LASIK And SBK

Objective To discuss the changes that occur in the posterior corneal elevation and high order aberration after femtosecond laser-assisted in situ keratomileusis（FS-LASIK）versus Sub-Bowmans keratomileusis（SBK）.Methods Prospective nonrandom control study.One hundred and seventy eyes of 85patients（170 eyes）who underwent LASIK for correction of myopia or myopic astigmatism were divided into FS-LASIK group（46 cases of 92 eyes）and SBK group（39 cases of 78 eyes）.Based on preoperative spherical equivalent,patients were further divided into moderate or low myopia group and high myopia group.All the patients were examined by Oculyzer before surgery and at 1 week,1 month and 3 months after surgery.The corneal high order aberration of anterior surface was measured,containing the Spherical aberration（C4⁰）,horizontal coma（C3¹）,vertical coma(C3^-1),horizonta trefoil（C3 ³）,oblique trefoil(C3^-3)and quadrafoil（C4⁴）.Posterior corneal elevation was measured at the corneal apex,thinnest corneal point and concentric circles with the diameter of 2mm,4mm and 6mm（central area,near peripheral area and far peripheral area）,26 points in total.We analyzed the changes in posterior corneal elevation and high order aberration at different time points in patients after two surgical procedures.The results were analyzed with repeated measures ANOVA analysis,paired t test and independent t test using SPSS 16.0 software.Results At postoperative1week,1 month and 3 months,the four groups showed generally consistent changing trends in posterior corneal elevation;corneal vertex,thinnest corneal point and central area slightly moved backwards,while the near and far peripheral areas slightly protruded forwards.At different postoperative time points and different parts,there was no statistically significant difference in posterior corneal elevation among the four groups（P>0.05）.In the FS-LASIK group,C3^-11 and C4⁰increased at the first and third month after operation compared with preoperative,and C3¹ decreased compared with that before the operation.The differences were statistically significant（P<0.01）.The C3¹、C4⁰ of third month was slightly less than that at 1 month after surgery,and the difference was statistically significant（t=3.81,4.25;P<0.01）.In the SBK group,the C3³,C3¹,and C4⁰ increased at the first and third month after operation compared with preoperative,and C4⁴ decreased compared with that before the operation.The differences were statistically significant（P<0.05）.The C4⁰ of third month was slightly less than that at 1 month after surgery,and the difference was statistically significant（t=3.91;P<0.01）.The difference of C3³,C3¹,C3^-1 between FS-LASIK and SBK groups were statistically significant at 1 month after operation（t=-2.71,-7.61,2.37;P<0.05）.The difference of C3¹,C3^-1 between two groups were also statistically significant at 3 month after operation（t=-6.32,2.84;P<0.05）.Even though the C4⁰ of two groups were both higher after operation,there was no significant difference between two groups（P>0.05）.Conclusions After FS-LASIK and SBK,the changes in posterior corneal elevation in moderate or low myopia groups and high myopia groups were small.The changing trends were as follows:central area slightly moved backwards,while peripheral area slightly protruded forwards.However,both operations can cause an increase in spherical aberration and coma.