Changes Of Posterior Corneal Elevation And Its Influencing Factors After Small Incision Lenticule Extraction For Myopia Correction

Objective:To compare the early changes in the posterior corneal elevation after correction of myopic by femtosecond laser small incision lenticule extraction（SMILE）,and to analyze the possible influencing factors.Methods:In this prospectivecohort study,the prospective cohort study included 37patients（73 eyes）with myopia and myopic astigmatism who underwent SMILE surgery at the Myopic Treatment Center of Shengjing Hospital Affiliated to China Medical University from October 2021 to October 2022.According to the equivalent spherical scope（SE）designed before surgery,36 eyes were divided into low-moderate myopia group（SE≤-6.00D）and 37 eyes were divided into high myopia group（SE<-6.00D<-12.50D）.The center point（PCE）,the thinnest point（PTE）,the thickest point（PME）of the posterior corneal elevation,and the central 2-mm area（MPE-2mm）of the posterior corneal elevation were measured by the Pentacam three-dimensional anterior segment analysis system before surgery,1 week,1 month,and 3 months after surgery,respectively mm),4-mm region（MPE-4mm）,and 6-mm region（MPE-6mm）.Biomechanical evaluation was also performed using Corvis ST.Statistical analysis of data SPSS 25.0 statistical software was used to compare the changes of posterior corneal elevation between the two groups at different time points after surgery by repeated measurement analysis of variance.Pearson correlation coefficient was used to evaluate the correlation between the changes of posterior corneal elevation and the changes of operative parameters and biomechanical parameters.Results:At 3-month follow-up,the safety index and effective index were 1.06 and 1.00in the low-moderate myopia group and 1.13 and 1.05 in the high-myopia group,respectively,and there was no statistical significance in the safety index and effective index between the two groups（P>0.05）.TheΔMPE-6mm of high myopia group was1.39±1.28μm and 1.36±1.12μm,respectively,one month and three months after surgery.ΔMPE-6mm in low-moderate myopia group was 0.77±1.10μm and 0.76±1.31μm,respectively,and the difference between the two groups was statistically significant（P=0.028,P=0.041）.In the high myopia group,ΔMPE-4mm was 0.44±1.07μm,0.96±1.28μm and 0.80±1.06μm at 1 week,1 month and 3 months after surgery,respectively,and the difference was statistically significant at each time point after surgery(F_time=5.459,P>0.05).TheΔMPE-4mm at 1 month after surgery was significantly higher than that at 1 week after surgery,the difference was statistically significant（P=0.006）,but there was no statistically significant difference among the remaining time points（P>0.05）.In the high myopia group,ΔMPE-6mm was（0.94±1.12）μm,1.39±1.28μm and 1.36±1.12μm at 1 week,1 month and 3 months after surgery,respectively,and the difference was statistically significant at each time point after surgery(F_time=4.418,P>0.05).TheΔMPE-6mm at 1 month and 3 months after surgery was significantly higher than that at 1 week after surgery,the difference was statistically significant（P=0.013,P=0.048）,but there was no statistically significant difference inΔMPE-6mm at 3 months and 1 month after surgery（P=0.991）.There were no significant differences inΔPCE,ΔPTE,ΔPME andΔMPE-2mm between the two groups（All P>0.05）.There was no significant difference at each time point in the low myopia group（P>0.05）.There were no significant differences inΔPCE,ΔPTE,ΔPME,ΔMPE-2mm in high myopia group at each time point after operation（P>0.05）.At 3months after surgery,ΔMPE-2mm had a low positive correlation with age（r=0.302,P=0.009）,and 3moΔDA Ratio and 3moΔIR had a low negative correlation（r=-0.379,P=0.001;r=-0.304,P=0.009）;At 3 months,ΔMPE-6mm had a low negative correlation with W-W（r=-0.369,P=0.001）,a moderate negative correlation with PSK（r=-0.531,P<0.001）,a moderate positive correlation with ASK（r=0.548,P<0.001）,and a positive correlation with PAD（r=0.382,P=0.016）,and theΔDA Ratio at 3 months showed a low positive correlation（r=0.425,P<0.001）,andΔIR at 3 months showed a low positive correlation（r=0.351,P=0.032）.Conclusion:The posterior corneal elevation in the peripheral area moved slightly forward in the early stage,and gradually recovered to the preoperative stage at 3 months,and the forward movement was more obvious in the high myopia group in the 6-mm region lower and moderate myopia group.The height of posterior corneal surface was correlated with age,percentage of cutting depth,corneal diameter,surface curvature of anterior and posterior cornea,the change in the ratio of the deformation amplitude and the change in the Integr.Radius.