Clinical Study On The Changes Of Corneal Biomechanical Properties Before And After Corneal Refractive Sugeries

Objective:To analyze and invistigate the corneal biomechanics of pre-and post-laser refractive surgeries at the early stage and to determine their influence factors.Method:In the first random and prospective study,89subjects with moderative myopia were recruited. There were36subjects in Epi-LASIK group,34subjects in LASIK group and19subjects in SBK group, respectively. The other random and control study included32subjects with high myopia and the control group consisted of18subjects with low or moderate myopia.The Goldmann correlated IOP, Corneal compensated IOP, corneal resistance factor, corneal hysteresis, TimeIn and TimeOut were obtained preoperatively and postoperatively. The difference between their values before and after surgery was gained by Pared t-test; Independent-Sample t-test was applied to analyse their difference and changings1week and1month postoperatively. The Pearson correlation was used to explore their influence factors.Results:1. In Epi-LASIK group, postoperative biomechanics differed significantly (P<0.01). The difference between IOPg, TimeIn, TimeOut, the difference between CH and CRF1week and1month was significantly different(t=-2.50, P=0.014; t=-2.49, P=0.015;t=-2.87, P=0.005;t=2.386, P=0.019).2. After LASIK, corneal biomechanical parameters differed significantly from those preoperatively (P<0.01); TimeOut, the difference TimeOut and TimeIn1week were statistically different from1month (t=-2.82,.P=0.006; t=-2.02, P=0.046).3. In SBK group, the difference in the parameters before and after surgery was statistically significant(P<0.01), unlike that between1week and1month(P>0.05). The differences in postoperative CRF, CH and TimeOut between high myopia and control group were significant statistically(t=-2.66, P=0.010; t=-5.07, P=0.000; t=-2.91, P=0.005), so were the differences in ΔCRF, ΔCH, ΔTimeOut(t=2.00, P=0.048; t=3.14, P=0.002;t=2.78, P=0.007).4. Significant difference existed in IOPcc,TimeIn,CH-CRF and TimeOut-TimeIn between Epi-LASIK and SBK lmonth(t=2.34, P=0.022;t=2.32, P=0.024; t=-2.44, P=0.018;t=-2.30, P=0.025), so did in ΔIOPg, ΔIOPcc, ΔCRF, ΔTimeln, Δ(CH-CRF) and Δ(TimeOut-Timeln)(P<0.05). Statistically difference in IOPcc between LASIK and SBK groups1week was found (t=-2.04, P=0.045), so was in IOPg, IOPcc, CH, TimeOut, CH-CRF and TimeOut-TimeIn1month(t=-2.40, P=0.019; t=-2.89, P=0.005; t=2.27,P=0.027; t=2.90, P=0.005; t=2.65, P=0.010; t=2.71, P=0.009).5. A good correlation did not exist in between biomechanical changes and AD in Epi-LASIK group(P>0.05), but exsited in between ΔCRF, ΔCH, ATimeOut and AD in LASIK group1week (r=0.419,P=0.005; r=0.386, P=0.010; r=0.447, P=0.002) and between ΔTimeOut and AD in SBK group1week(r=0.385, P=0.039); At1month, the correlations were not obvious (P>0.05). In high myopia, there was a positive correlation between ΔIOPg, ΔCRF, ATimeln and AD (r=0.335, P=0.011; r=0.427, P=0.001;r=0.338, P=0.010) and a negative correlation between A(CH-CRF), Δ(TimeOut-TimeIn) and AD (r=-0.286,P=0.031; r=-0.306, P=0.020); However, a positive correlation was found between ATimeOut and AD (r=0.548, P=0.003) in the control group.Conclusions:1. Due to different refractive surgeries and ablation depth, different changes in corneal biomechanics happened:the LASIK procedure weakened the corneal biomechanical changes greater than SBK, SBK greater than Epi-LASIK.2. The postoperative parameters gradually recovered with time in Epi-LASIK group, but their changes were not obvious in LASIK and SBK groups postoperatively, which maybe relate to different corneal healing response.3. According to the correlation between AD and biomechanical changes at different time in Epi-LASIK, LASIK and SBK, we can know that the surgical procedure and AD afftected the corneal biomechanics more obviously1week than1month and that lamellar ablation weaken corneal biomechanics more than surface ablation, thin-flap lamellar ablation more than thick-flap ablation.4. Compared to the control group, the procedure for high myopia more substantially weakened corneal properties and induced to greater changes about corneal biomechanic singnals, which related to ablation depth to some extent.