Changes in corneal biomechanics and posterior corneal surface elevation after FS-LASIK

AIM:To investigate the changes in corneal biomechanics and posterior corneal surface elevation after femtosecond laser-assisted in situ keratomileusis(FS-LASIK).METHODS:Totally 197 eyes of 100 patients who underwent the FS-LASIK from April 2022 to November 2022 were included.They were divided into three groups according to the ratio of residual corneal stroma thickness/corneal thickness(RCST/CT):Group I(50%≤RCST/CT<55%,63 eyes of 32 patients),Group II(55%≤RCST/CT<60%,67 eyes of 34 patients),and Group III(RCST/CT≥60%,67 eyes of 34 patients).The intraocular pressure(IOP),corneal compensated IOP(IOPcc),corneal hysteresis(CH)and corneal resistance factor(CRF)were measured immediately,1,and 3mo postoperatively by ocular response analyzer(ORA)and the posterior elevation difference(PED)was measured by Pentacam.RESULTS:After operation,IOP,CH,CRF,and PED were statistically different among the three groups(F=12.99,31.148,23.998,all P<0.0001).There was no statistically significant difference in IOPcc among the three groups(F=0.603,P>0.05).The IOP,IOPcc,CH,and CRF were statistical changed after surgery(F=699.635,104.125,308.474,640.145,all P<0.0001).The PED of Group I was significantly higher than that of Group II(P<0.05),and Group II was significantly higher than that of Group III(P<0.05).The PED value of 3mo after surgery decreased in each group compared with 1mo after surgery,but there was no statistical difference(Group I:t=0.82,P=0.41;Group II:t=0.17,P=0.87;Group III:t=1.35,P=0.18).The correlation analysis of corneal biomechanical parameter changes with PED at 1mo and 3mo after surgery showed thatΔIOP,ΔIOPcc,ΔCH,andΔCRF were not correlated with PED value in three groups(P>0.05).CONCLUSION:The smaller the RCST/CT,the greater effect on corneal biomechanics and posterior surface elevation.There is no correlation between changes in corneal biomechanics and posterior corneal surface elevation in the range of RCST/CT≥50%.

Unintended changes in ocular biometric parameters during a 6-month follow-up period after FS-LASIK and SMILE

Background:Corneal refractive surgery has become reliable for correcting refractive errors,but it can induce unintended ocular changes that alter refractive outcomes.This study is to evaluate the unintended changes in ocular biometric parameters over a 6-month follow-up period after femtosecond laser-assisted laser in situ keratomileusis(FS-LASIK)and small incision lenticule extraction(SMILE).Methods:156 consecutive myopic patients scheduled for FS-LASIK and SMILE were included in this study.Central corneal thickness(CCT),mean curvature of the corneal posterior surface(K_(pm)),internal anterior chamber depth(IACD)and the length from corneal endothelium to retina(ER)were evaluated before and after surgery over a 6-month period.Results:Both the FS-LASIK and SMILE groups(closely matched at the pre-surgery stage)experienced flatter Kpm,shallower IACD and decreased ER 1 week post-surgery(P<0.01),and these changes were larger in FS-LASIK than in SMILE group.During the 1 week to 6 months follow up period,K_(pm),IACD and ER remained stable unlike CCT which increased significantly(P<0.05),more in the FS-LASIK group.Conclusions:During the follow up,the posterior corneal surface became flatter and shifted posteriorly,the anterior chamber depth and the length from the corneal endothelium to retina decreased significantly compared with the pre-surgery stage.These unintended changes in ocular biometric parameters were greater in patients undergoing FS-LASIK than SMILE.The changes present clear challenges for IOL power calculations and should be considered to avoid affecting the outcome of cataract surgery.