Study On Inverse Finite Element Method For Measuring Corneal Elastic Parameters In Vivo

Myopia is a common eye disease.With the development of corneal refractive surgery,more and more patients have improved their vision through surgery.However,refractive surgery has risks for some individuals.The procedure removes a portion of the corneal stroma and alter the refractive power of the cornea by altering the geometry of the cornea.Under the effect of IOP(intraocular pressure)in the anterior chamber fluid,the cornea will reach a new physiological equilibrium state after thinning.Therefore,the physiological balance of the cornea changes,and the biomechanical properties of the cornea have a critical impact on postoperative outcomes.The experimental evaluation of the mechanical properties of the cornea is of great significance for predicting postoperative corneal deformation and controlling the risk of postoperative complications.The non-contact tonometer CorVisST(corneal visualization Scheimpflug technology)uses a short air jet to impact the central region of the cornea and uses a highspeed camera to record corneal deformation.It not only provides more accurate intraocular pressure measurements,but also has the potential to be used as an in vivo test to assess corneal mechanical properties.In this paper,the anisotropic hyperelastic constitutive theory describing the distribution of corneal fibers was used to characterize the hyperelastic mechanical behavior of in vivo corneal tissue materials.The finite element numerical formula was derived and the ABAQUS user material subroutine was written to apply the constitutive model to finite element calculation which provided a basis for subsequent research.We used the structural analysis and fluid-structure interaction to simulate the measurement process of CorVisST respectively.The influence of the simulation method of the air-cornea interface on the simulation results was discussed and a simulation method combining digital image processing and FLUENT/UDF function to define the dynamic boundary of the flow field was proposed.The dynamic response of the cornea under the fixed boundary condition and the moving boundary condition was simulated.The coupling effect of the eye movement on the corneal deformation was discussed.Finally,material parameters of different constitutive models of the cornea were extracted by the inverse solution finite element method based on air puff test.We explored the effects of corneal mechanical properties on corneal deformation and evaluated the feasibility of this approach for extracting corneal material parameters.Studies have shown that fluid-structure interaction is important for accurately simulating measurement process of CorVisST.Corneal deformation has a significant effect on the flow field.The linear spring-linear damper system does not simulate eye movement well,the system has no significant effect on corneal deformation.The corneal nonlinearity and fiber distribution have a significant influence on the corneal deformation profile,which may help to construct a suitable inverse solution strategy when using finite element inverse solution to solve corneal material parameters,and has potential to obtain individualized differences in corneal matrix shear modulus and fiber stiffness.