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

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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.