Study On The Hyperelastic Behaviors Of Corneal Materials Considering The Effect Of Refractive Surgery And Corneal Cross-linking

With the increasing popularity of refractive surgery,refractive errors of the myopic patients can be effectively improved.However,the degradation of the biomechanical properties of corneas after the refractive surgery induces the complication of corneal ectasia,which seriously threatens the safety of postoperative patients.Corneal cross-linking(CXL)has been clinically proven to be able to improve the strength and stability of corneal stroma for the treatment of corneal ectasia.In this dissertation,both theoretical modeling and numerical simulation are combined to study the mechanical behaviors of corneal materials after refractive surgery and CXL treatments.The following results are obtained:(1)Considering the strengthening mechanisms of the matrix,fibers and cross-links as well as the softening behaviors induced by the damage of fibers and cross-links,a hyperelastic model was established considering cross links and microstructure damage.By comparing the model results with the experimental data of uniaxial tension-failure tests for corneal lens strips,the universality and accuracy of the constitutive model have been verified.The results indicate that corneal strength increases with the radiation dose.Under super-physiological loads,the damage of fibers and cross-links is the main cause resulting in the degradation of corneal material strength and even failure.(2)A corneal hyperelastic model considering the gradient distribution of cross-links is developed,which can effective characterize the spatial non-linear distribution of cross-links.Moreover,the constitutive equations are implemented into the UMAT subroutine of ABAQUS to help simulate the corneal inflation tests.The established finite element simulation model can well address the mechanical behavior of corneas with different CXL treatments.The simulated results are in good agreement with the experimental data,including the corneal intraocular pressure-apex displacement relations,which can help comprehend the underlying deformation mechanisms with the effect of CXL.(3)A finite element simulation framework for the corneal indentation tests is established to address the effects of refractive surgery and CXL therapy on the biomechanical properties of cornea.The results show that CXL therapy can effectively improve the mechanical properties of cornea after refractive surgery.Meanwhile,the optimal design of refractive surgery is carried out considering the influence from the aspect of biomechanics.It is noticed that the small incision for refractive surgery should be 45° with respect to the NT direction,the radian is 20-40°,and the lateral incision angle should be perpendicular to the upper surface of the cornea.In addition,the corneal cap should be retained as much as possible on the premise of ensuring corrected corneal vision.