Three-Dimensional Characterization and Modulation of Corneal Biomechanics via Brillouin Microscopy

Abstract

Corneal mechanical properties are needed for diagnosing and monitoring the progression of ocular disorders such as keratoconus, screening for refractive surgeries, and evaluating treatment procedures including corneal cross-linking. Alterations of these mechanical properties are often localized to a specific area within the cornea. However, there exists a clinical gap of measuring local mechanical properties as current methods are contact-based and provide global measurements. The goal of this dissertation is to close this gap by establishing a three-dimensional, noninvasive characterization method of corneal biomechanics. Previously, our laboratory developed Brillouin microscopy as an imaging modality which can noninvasively extract mechanical measurements of a material. Here, using Brillouin microscopy, we characterized the stiffening effects of accelerated and localized cross-linking procedures with three-dimensional resolution. However, existing procedures to extract elastic modulus information from Brillouin measurements rely on empirical calibrations because a fundamental understanding between the two had not yet been established. In practice, this limits Brillouin measurements to relative softening / stiffening information, which, while useful to compare protocol efficacies, are not optimal for modeling long-term shape behavior of the cornea in clinical settings. Here, we address this shortcoming of Brillouin microscopy. First, we identified that both Brillouin-derived mechanical modulus and traditional elastic modulus are dependent on two major biophysical factors: hydration and the mechanical properties of the solid matrix. We derived and experimentally verified a quantitative relationship to describe the distinct moduli dependencies of such factors. Based on these relationships, we derived a procedure to extract the elastic modulus of the cornea from experimental measurements of Brillouin frequency shift and hydration, two clinically available parameters. Thus, the work presented here establishes a spatially resolved, noninvasive method for measuring corneal elastic modulus.