Analysis Of The Corneal Biomechanics Of Myopic Eyes And Its’ Influencing Factors After Small Incision Lenticule Extraction

Purpose To analyze the corneal biomechanics in terms of corneal deformation behavior and discover the attribution, the influencing factors and normal ranges of corneal deformation parameters. To investigate on the changes of corneal biomechanics after small incision lenticule extraction(SMILE) and the influencing factors.Methods Firstly, the bservational study. Four hundred and seventy-seven eyes of 257 myopic patients were included. The values of first applanation time(1st A-time), first applanation(1st A-length), first applanation velocity(1st A-velocity), second applanation time(2nd A-time), second applanation length(2nd A-length), second applanation velocity(2nd A-velocity), highest concavity time(HC-time), highest concavity deformation amplitude(HC-DA), highest concavity peak distance(HC-Peak Distance), highest concavity radius(HC-Radius), intraocular pressure(IOPnct) and the central corneal thickness(CCTst) were measured using the Corvis ST. The distributions of the data were tested with the Shapiro-Wilk test. The Pearson correlation coefficient(r) and spearman correlation coefficient(rs) were used to evaluate the relationship between variables. Multiple linear regressions were used to analyze the relationship between the 1st A-time, 2nd A-time, and HC-DA with the biomechanical parameters and corneal morphology. Then the serial cases-observional study was performed. One hundred eyes of 58 patients underwent SMILE surgery were included in this study. The values of CRF(corneal resistance factor) and CH(cornea hysteresis) were measured using the ocular response analyzer(ORA) at preoperative, 1 month, 3 months, 6 months and 1 year postoperatively. The comparisons among different follow-up periods were conducted by the repeated measures analysis of variance(RMANOVA). The comparison between every two periods was conducted by the Sidak test. Grouped the participants by the preoperative CRF and RST, the comparisons among different follow-up times were conducted by the one-way analysis of variance(one-way ANOVA). The comparison between every two groups was conducted by the Sidak test. The Pearson correlation coefficient(r)was used to evaluate the linear fit of the relationship between variables.Results 1. The 1st A-time, 2ndA-time, HC-time, HC-DA, CCTst and IOPnct are normally distributed, however, other corneal deformation parameters are skewness distributed. 2. The corneal resistance factor(CRF) was correlated with all the parameters of Corvis ST except for the 1st A-length(p<0.05), CH were correlated with 1st A-time, 2nd A-length, 2nd A-velocity, HC-time, HC-Radius and HC-Peak Distance(p<0.05), all the parameters except for the 1st A-length are correlated with the CCT measured by the Pentacam(p<0.05), the IOPcc was correlated with all the deformation parameters excluding the 1st A-length, 2nd A-length and HC-time. The steep curvature(K2) was in correlation with 1st A-length, 1st A-velocity, 2ndA-time, HC-DA and HC-Radius(p<0.05). 3. The CCTst was significantly smaller than the CCT, and the IOPst was also smaller than the IOPg(the Godmann intraocular pressure) and the IOPcc measured by the ORA(p<0.05). 4. In the Multiple linear regressions, the IOPcc and CRF were introduced to the regression equation of 1st A-time, 2nd A-time and HC-DA(p<0.05). 5. The CRF and CH at 1 month after the SMILE were significantly lower than the preoperative CRF and CH(p<0.05). For the CRF and CH values, no significant differences were found between each period and its former period from 1-month postoperatively(p>0.05). 6. The ΔCRF(Δ=preoperative value- 1-year postoperative value) and ΔCH were negatively correlated with preoperative spherical equivalent diopter and the sum of preoperative spherical diopter and cylindrical diopter(p<0.05), positively correlated with preoperative CRF and preoperative CH p<0.05), positively correlated with the lenticule thickness(LT) and the ratio of the LT and the preoperative CCT(p<0.05). Meanwhile, the ΔCRF and ΔCH were positively correlated with the ratio of postoperative RST and preoperative CCT(p<0.05). No correlation was found between the ΔCRF or ΔCH with the age and the preoperative mean curvature(Km)(p>0.05). 7. The range of RST was 288~412μm in this study, no statistical difference ofCRF and CH values was found between different RST groups(p>0.05).Conclusion 1. In the corneal deformation parameters, the average of the 1st A-timeis(7.39±0.19)ms, the average of the 2nd A-time is(21.76±0.30)ms, and the average of HC-DA(1.08±0.08)mm, which could be beneficial for doctors. 2. The corneal deformation parameter is the combined reflection of intraocular pressure, central corneal thickness, corneal curvature and corneal biomechanics, and the most important function is the description of corneal biomechanics. 3. The corneal deformation response could be affected by intraocular pressure greatly, thus the IOP must be taken into consideration when we evaluate corneal biomechanics in clinics. 4. The reliability of the Corvis ST in the measurement of central corneal thickness and intraocular pressure remains to be further studied. 5. The small incision lenticule extraction could weaken the corneal biomechanics, but the stability of corneal biomechanics could obtain at 1 month postoperative. 6. For the SMILE surgery, certain safety could be guaranteed from the aspect of corneal biomechanics if the RST is no less than 288μm. 7. The higher the ratio of the LT with the preoperative CCT, the changes of CRF and CH would be larger. The main factor affecting the SMILE corneal biomechanics is the lenticule thickness and corrected-diopter except for the preoperative CRF and CH.