Three-dimensional, biomechanical characterization of customized corneal collagen crosslinking using Brillouin microscopy.

Abstract

Purpose : Customized corneal collagen crosslinking (CXL) has recently been proposed to address the spatially-dependent characteristics of ectasia. In principle, locally stiffening the cornea would provide greater topographical flattening and quicker recovery post-CXL while using less UV-A energy. However, the biomechanical properties of the cornea after localized CXL procedures have not yet been evaluated as no method has noninvasively and spatially measured the resulting corneal stiffness. Here, we utilize Brillouin microscopy to characterize the spatially-varying stiffness of the cornea following customized CXL.

Methods : Porcine eyes were obtained from a local slaughterhouse within 2 hours of death. Localized crosslinking protocols were mimicked by soaking extracted cornea samples in riboflavin solution for 30 minutes and applying a spatially-varying light dose across the tissue. Several gradients in light intensity exposure were obtained using blocking masks and intensity filters, with the largest light dose (5.4 J/cm²) featuring 9 mW/cm² UV-A radiation for 10 minutes. Following localized CXL, the stiffness of each sample was mapped via Brillouin microscopy.

Results : After localized CXL, three distinct regions were identified in the cornea samples: a maximal crosslinked area, corresponding to the expected stiffness changes following the 9 mW/cm² irradiation protocols; a minimal, separately crosslinked section, corresponding to the stiffness changes following the crosslinking protocol of lesser energy; and, in between, a transition zone with spatially-varying corneal stiffness. The extent of the transition zone and its stiffness variation were highly dependent on the crosslinking protocol and configuration. Validating with stress-strain mechanical tests, we calculated Brillouin microscopy is able to detect as small as a 0.06% relative change in Young’s Modulus with three-dimensional, micron-level resolution.

Conclusions : Customized, local corneal collagen crosslinking protocols produce spatially-varying stiffness patterns in three-dimension. As a result, spatially-resolved mechanical characterizations, such as that demonstrated with Brillouin microscopy, will be necessary to evaluate the mechanical and long-term refractive outcomes following localized CXL.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.