Study On The Effects Of Surgery And Pathology On Corneal Biomechanical Properties Based On ORA And Corvis ST

Cornea is an important part of human refractive system.Its biomechanical properties play an important role in maintaining the shape of cornea and ensuring the normal refractive power of the eyeball.The corneal ablation by refractive surgery not only destroys the corneal integrity,but also affects the corneal biomechanical properties,and may lead to secondary keratoconus after surgery.The idiopathic keratoconus and secondary keratoconus after refractive surgery will change the shape of cornea and seriously affect the visual acuity of patients.Therefore,noninvasive detection and accurate evaluation of corneal biomechanical properties in vivo are of great significance for clinicians to assess the risk of refractive surgery and diagnose early keratoconus lesions.By collecting the clinical data?including ORA and Corvis ST?of normal people,keratoconus patients and myopic patients before and after LASIK,the overall mechanical properties of cornea and the biomechanical properties of corneal materials were analyzed.In order to explore the effects of surgery and pathological changes on corneal biomechanical properties.The main research work and conclusions are as follows:1.Corneal hysteresis?CH?and corneal resistance factor?CRF?were recorded and compared before and after LASIK by Ocular Response Analyzer?ORA?.The results showed that CCT,CH,CRF,CH/CCT,CRF/CCT decreased statistically after LASIK compared with those before LASIK,indicating that the overall corneal stiffness and viscosity decreased with the decrease of corneal thickness after LASIK.At the same time,it was found that CRF_beforeefore and CH_beforeefore were correlated with CT_beforeefore and IOPcc?P<0.05?;CRF_afterfter and CH_afterfter were correlated with dCCT?P<0.05?;dCRF and dCH were correlated with DCCT?P<0.05?.2.The data provided by Corneal Visualization Scheimpflug Technology?Corvis ST?were analysised,and the relationship between pulse pressure and vertex displacement was determined.The indicators of corneal elasticity(S_TSC)and viscosity(A_absorbed),were obtained from the images respectively.The S_TSCSC and A_absorbedbsorbed before and after LASIK were recorded and compared.The results showed that CCT,S_TSC,A_absorbed,S_TSC/CCT and A_absorbed/CCT decreased statistically after LASIK compared with those before LASIK,indicating that the overall corneal stiffness and viscosity decreased with the decrease of corneal thickness after LASIK.At the same time,it was found that dS was correlated with bIOP_before,dCCT and DCCT?all P<0.05?;dA was only correlated with A_before.?P<0.01?.3.The results of ORA and Corvis ST were compared and analyzed.It was found that the results of the two methods were consistent,that is,the corneal stiffness and viscosity decreased after LASIK.Predictive formulas of corneal S_TSCSC before surgery(S_before)and S_TSCSC after surgery(S_after)were obtained by multiple linear regression analysis:S_before=1.055bIOP_before+0.015CCT_before,?F=2706.33,P=0.000,R²=0.986?;S_after=0.937S_before+0.019CCT_after,?F=1821.99,P=0.000,R²=0.976?.4.The difference of biomechanical properties between normal cornea and keratoconus was analyzed by pulse pressure-vertex displacement curve.It was found that compared with the normal cornea,the CCT,bIOP,S_TSCSC and S_TSC/CCT of keratoconus decreased,while the A_absorbedbsorbed and A_absorbed/CCT increased,the difference was statistically significant.This shows that compared with normal cornea,the stiffness of keratoconus decreases while the viscosity increases.5.Using standard linear solid model,the viscoelastic coefficients of normalcornea and keratoconus were calculated by two methods?multi-parameter fitting method and Simulink simulation method?,and the differences between the viscoelastic coefficients of keratoconus and normal cornea were compared.The results show that the standard linear solid model can characterize the corneal viscoelasticity to a certain extent,and the above ideas have a good effect on the solution of corneal viscoelasticity coefficient.It was found that the elastic coefficients and viscoelastic coefficients of keratoconus were larger than those of normal cornea,and the difference was statistically significant.And the difference between the two methods is not statistically significant.In the multi-parameter fitting method,the ROC curve shows that the area under the curve of corneal elasticity coefficient,viscous coefficient and their combined indexes are 0.776,0.895 and 0.948,which shows that the results obtained by this method are of high accuracy in the diagnosis of keratoconus.6.The overall biomechanical properties and material mechanical properties of normal cornea and keratoconus were compared and analyzed.The results showed that the elasticity coefficient of keratoconus was increased compared with that of normal cornea,but the overall stiffness of keratoconus decreased with the decrease of its thickness,which resulted in the decrease of its ability to resist deformation.