Changes in posterior corneal elevation after small incision lenticule extraction for different myopic diopters

●AIM:To study the changes and effect factors of posterior corneal surface after small incision lenticule extraction(SMILE)with different myopic diopters.●METHODS:Ninety eyes of 90 patients who underwent SMILE were included in this retrospective study.Patients were allocated into three groups based on the preoperative spherical equivalent(SE):low myopia(SE≥-3.00 D),moderate myopia(-3.00 D>SE>-6.00 D)and high myopia(SE≤-6.00 D).Posterior corneal surfaces were measured by a Scheimpflug camera preoperatively and different postoperative times(1wk,1,3,6mo,and 1y).Posterior mean elevation(PME)at 25 predetermined points of 3 concentric circles(2-,4-,and 6-mm diameter)above the best fit sphere was analyzed.●RESULTS:All surgeries were completed uneventfully and no ectasia was found through the observation.The difference of myopia group was significant at the 2-mm ring at 1 and 3mo postoperatively(1mo:P=0.017;3mo:P=0.018).The effect of time onΔPME was statistically significant(2-mm ring:P=0.001;4-mm ring:P<0.001;6-mm ring:P<0.001).The effect of different corneal locations onΔPME was significant except 1wk postoperatively(1mo:P=0.000;3mo:P=0.000;6mo:P=0.001;1y:P=0.001).Posterior corneal stability was linearly correlated with SE,central corneal thickness,ablation depth,residual bed thickness,percent ablation depth and percent stromal bed thickness.●CONCLUSION:The posterior corneal surface changes dynamically after SMILE.No protrusion is observed on the posterior corneal surface in patients with different degrees of myopia within one year after surgery.SMILE has good stability,accuracy,safety and predictability.

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%.

Impact of ablation ratio on 5-year postoperative posterior corneal stability after refractive surgery:SMILE and FS-LASIK

Background:To investigate the impact of the ablation ratio on 5-year postoperative posterior corneal stability in myopic eyes after small incision lenticule extraction(SMILE)and femtosecond laser-assisted in situ keratomileusis(FS-LASIK)surgery.Methods:A prospective,nonrandomized,cohort study:80 eyes of 43 patients underwent SMILE surgery and 63 eyes of 32 patients underwent FS-LASIK surgery at the EYE&ENT Hospital,Fudan University.Ablation ratio Was defined as lenticule thickness(SMILE cases)or ablation depth(FS-LASIK cases)divided by central corneal thickness(CCT).Posterior corneal elevation changes were recorded as posterior central elevation(PCE),posterior corneal surface at thinnest point(PTE)and posterior corneal mean elevation(PME).Patients were followed up at 6-month and 5-year intervals to investigate the impact of the ablation ratio on posterior corneal elevation after SMILE and FS-LASIK surgery.Results:PCE dropped at the 6-month follow-up for both SMILE(decreased by-1.11±2.93μm,P<0.05)and FSLASIK groups(decreased by-0.46±3.72μm,P<0.05).PTE also dropped in SMILE(reduced by-2.04±3.02μm,P<0.05)and FS-LASIK group(reduced by-1.28±4.21μm,P<0.05)at the 6-month follow-up.Stable PCE(elevation change:SMILE-0.28±4.03μm;FS-LASIK 0.79±4.13μm,P>0.05)and PTE(elevation change:SMILE-0.08±4.28μm;FS-LASIK 1.42±3.85μm,P>0.05)for both groups were recorded at the 5-year follow-up compared to the 6-month visit.Ablation ratio was strongly correlated with 5-year postoperative PCE(β=2.68±1.05,P<0.01)and PTE(β=2.35±1.17,P<0.05).Cut-off value raised for 5-year postoperative PCE and PTE was 27.3%and 27.1%,respectively.Conclusions:Ablation ratio was strongly correlated with postoperative posterior corneal elevation in a 5-year follow-up in both SMILE and FS-LASIK groups.PCE and PTE underwent slight backward displacement 6 months postoperatively and remain stable at the 5-year follow-up.Threshold of the ablation ratio for resisting forward displacement of posterior corneal surface Was 27.3%and 27.1%for SMILE and FS-LASIK groups,respectively.

Impact of ablation ratio on 5-year postoperative posterior corneal stability after refractive surgery:SMILE and FS-LASIK

Background:To investigate the impact of the ablation ratio on 5-year postoperative posterior corneal stability in myopic eyes after small incision lenticule extraction(SMILE)and femtosecond laser-assisted in situ keratomileusis(FS-LASIK)surgery.Methods:A prospective,nonrandomized,cohort study:80 eyes of 43 patients underwent SMILE surgery and 63 eyes of 32 patients underwent FS-LASIK surgery at the EYE&ENT Hospital,Fudan University.Ablation ratio was defined as lenticule thickness(SMILE cases)or ablation depth(FS-LASIK cases)divided by central corneal thickness(CCT).Posterior corneal elevation changes were recorded as posterior central elevation(PCE),posterior corneal surface at thinnest point(PTE)and posterior corneal mean elevation(PME).Patients were followed up at 6-month and 5-year interval to investigate the impact of the ablation ratio on posterior corneal elevation after SMILE and FSLASIK surgery.Results:PCE dropped at the 6-month follow-up for both SMILE(decreased by−1.11±2.93μm,P<0.05)and FSLASIK groups(decreased by−0.46±3.72μm,P<0.05).PTE also dropped in SMILE(reduced by−2.04±3.02μm,P<0.05)and FS-LASIK group(reduced by−1.28±4.21μm,P<0.05)at the 6-month follow-up.Stable PCE(elevation change:SMILE−0.28±4.03μm;FS-LASIK 0.79±4.13μm,P>0.05)and PTE(elevation change:SMILE−0.08±4.28μm;FS-LASIK 1.42±3.85μm,P>0.05)for both groups were recorded at the 5-year follow-up compared to the 6-month visit.Ablation ratio was strongly correlated with 5-year postoperative PCE(β=2.68±1.05,P<0.01)and PTE(β=2.35±1.17,P<0.05).Cut-off value for 5-year postoperative raised PCE and PTE was 27.3 and 27.1%,respectively.Conclusions:Ablation ratio was strongly correlated with postoperative posterior corneal elevation in a 5-year follow-up in both SMILE and FS-LASIK groups.PCE and PTE underwent slight backward displacement 6-month postoperatively and remain stable at the 5-year follow-up.Threshold of the ablation ratio for resisting forward displacement of posterior corneal surface was 27.3 and 27.1%for SMILE and FS-LASIK groups,respectively.