Thesis (Ph. D.)--University of Rochester. Department of Mechanical Engineering, 2019.
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 goal of this thesis
work is aimed towards investigating the interaction of corneal biomechanical and optical
behaviors 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. The goal of the thesis can be divided into four specific aims.
First, we investigated the biomechanical and optical behaviors of a healthy
normal cornea at various IOPs through numerical simulations based on a widely
accepted anisotropic hyperelastic FE model. We conducted a sensitivity analysis based
on a powerful experimental/statistical technique, the DOE method. The biomechanical
and optical responses of the cornea to IOP elevation as well as the relative contribution
of multiple geometrical and material parameters to corneal biomechanical and optical
behaviors were evaluated. We found that the radius of curvature of the cornea was the
most important geometric parameter that contributes to both biomechanical and optical
behaviors of the cornea. For material parameters, corneal apical displacement was
influenced nearly evenly by matrix stiffness, fiber stiffness and nonlinearity. However, the
corneal optical aberrations were primarily affected by the matrix stiffness and the
distribution of collagen fibril dispersion. These findings have important implications for
future theoretical and experimental studies of the cornea, especially for the development
of an individualized cornea model.
Second, we proposed new methods for material characterization of individual
corneas. We aimed to characterize a complete set of material parameters for developing an individualized 3-D anisotropic hyperelastic corneal model, which provides accurate prediction of the interrelation between corneal biomechanics and optics of a specific
cornea. We proposed novel methods mainly focusing on the individual quantification of
three challenging material parameters, including collagen fiber stiffness, collagen fiber
nonlinearity and collagen fibril dispersion using optical information of the cornea to
overcome the traditional challenges in corneal material characterization. The new
material characterization method could also be beneficial for future development of an in
vivo individualized biomechanical model of the cornea and the investigation of the
impact of corneal biomechanics on patient’s visual performance for clinical applications.
Third, we evaluated the clinical significance of corneal biomechanical modeling in
one of the important clinical applications, laser refractive surgery. An accurate prediction
of the biomechanical response of the cornea to tissue ablation would help to predict
postoperative surgical outcomes, which can be taken into account in developing new
surgical paradigms for obtaining optimal surgical outcomes. The predictive ability of our
biomechanical model was evaluated by simulating myopic corrections in PRK surgery.
Our findings suggest that both of the spatial variation in collagen fibril dispersion and the
depth-dependent extrafibrillar matrix stiffness play a significant role in the postoperative
biomechanical and optical outcomes. Characterization of these two material features
helps to predict more accurate trend of the HOAs induced by the surgical process.
Lastly, we explored a novel method to induce in vivo IOP elevation for potential
future development of an in vivo corneal model. Our new material characterization
methods require a measurement of corneal optical behavior at varied IOP levels.
Therefore, we investigated the potential of developing an in vivo individualized corneal model for clinical applications by developing an efficient and non-contract method to control IOP elevation in vivo. For the first time, we showed that in vivo IOP can be
temporarily elevated and controlled in an innovative, safe, non-contact way using an
inversion table.
The research presented in this thesis helps to gain understandings of the
biomechanical and optical responses of individual corneas to various intraocular
pressures and to corneal surgery, such as laser vision correction. Furthermore, the
capabilities and techniques described in the thesis may be applied to investigate
underlying mechanisms, diagnosis and treatments of other clinically important
ophthalmic pathologies such as keratoconus, post-refractive ectasia and glaucoma.
