Investigation of biomechanical and optical properties of the human cornea by developing an individualized finite element model

Mengchen Xu, Advised by Prof. Geunyoung Yoon and Prof. Amy Lerner

Hopeman 224

The biomechanics of the cornea has a significant impact on its optical behavior. Alterations in corneal biomechanics lead to abnormalities in the surface topography and affect ocular aberrations that degrade retinal image quality. The work in this thesis is aimed towards investigating the interaction of corneal biomechanical and optical behavior through development of an individualized corneal model based on the finite element method that accounts for the large variations in corneal geometry and material properties. Our preliminary studies using the model investigated the optical and mechanical response of human cornea to different intraocular pressure (IOP) and refractive surgery procedure. A simplified axisymmetric corneal model with hyperelastic material developed first was upgraded by incorporating more complicated anisotropic material characteristics including fibril distribution and dispersion into a three-dimensional (3-D) model. Sensitivity analysis performed in this advanced model identified matrix stiffness as the most critical material parameter that affects both biomechanical and optical behavior of the cornea. Second, we evaluated the potential of the 3-D corneal model for laser refractive surgery that further took depth-dependent matrix stiffness into account. It was demonstrated that surgical outcomes in terms of corneal optics predicted by the model are strongly correlated with clinically observed trends.  To further improve our understanding, additional research with two specific aims is proposed as the next step to complete the thesis. The first aim is to quantify individual corneal material properties. Specifically, the 3-D corneal model developed previously will be validated from ex vivo inflation and shear tests to measure fibril dispersion distribution and depth-dependent matrix stiffness. Followed by the validation, we propose a customized method, combining ex vivo experimental measurements with sensitivity analysis to quantify the remaining material properties of a specific human cornea. The second aim is to simulate laser refractive surgery experimentally based on individual material properties of each human donor cornea and the results will be compared with model prediction. In conclusion, completion of the research proposed in this thesis will improve our scientific knowledge of the biomechanical and optical behavior of both non-surgical and surgical corneas and may offer a new method to quantify the material properties of human cornea. This achievement makes it possible to develop a patient-specific biomechanical corneal model that can reduce variability in optical outcomes of laser refractive surgery.