Analysis of experimental IOP-induced scleral deformations at the sub-micrometer scale using electronic speckle interferometry

Purpose: We developed and validated a new methodology for computing sub-micrometer scale IOP-induced deformation obtained during pressurization testing of human scleral shells. The overall goal is to elucidate the role of ocular biomechanics in the development of glaucoma by constructing eye-specific finite element models of the posterior pole and ONH. This work focused on establishing a system to evaluate the range of nonlinear, anisotropic biomechanical properties of human posterior sclera for inclusion in those models. Methods: One human posterior sclera shell was loaded on a custom pressurization apparatus. IOP was raised from 4.81 mmHg to 45.46 mmHg while keeping the tissue immersed in balanced salt solution (PBS). The resulting scleral deformations were optically recorded using a commercial Electronic Speckle Pattern Interferometer (ESPI). A validation test was performed on a spherical balloon to simulate the mechanical behavior of the sclera shell while immersed both in air and in PBS. The validation test was used to develop a correction algorithm for the scleral displacement errors induced by placing a PBS bath into the optical path of the interferometer. The correction methodology compensated for both the change in laser wavelength and the bending of the laser beams induced by the refraction of the PBS. Results: Displacement fields of the rubber balloon highlight the repeatability of the inflation test for the specimen immersed in both air and in PBS. The analysis procedure we developed successfully accounted for the diffractive effects caused by immersing the specimen in the PBS bath. Conclusions: The interferometric data analysis procedure has been shown to provide a reliable computation of submicrometer IOP-induced scleral deformation obtained from mechanical tests on human sclera shells immersed in saline, which is a fundamental requirement for investigating the IOP-related biomechanical behavior of the posterior pole.