Content area
Abstract
Glaucoma is an ocular disease characterized by the progressive loss of the retinal ganglion cell axons within the optic nerve head (ONH). It is the second cause of blindness worldwide. Yet, the precise mechanisms leading to vision loss remain unknown. A working hypothesis asserts that the damage to the axons is initiated by excessive level of intraocular pressure related stresses and strains in the tissues of the ONH. The deformation of the ONH is determined by the mechanical properties of the ONH and scleral tissue. The objective of this work is to develop experimental and modeling tools to characterize the anisotropic viscoelastic properties of scleral tissue of normal and glaucoma human donors, evaluate the relationship between scleral collagen structure and mechanical properties of the sclera, and determine the effects of scleral collagen structure on the ONH deformation.
We designed an inflation test to measure the pressure-induced full-field deformations of the posterior sclera from 22 donors with no history of glaucoma and 11 donors with a history of glaucoma under close to physiological conditions. We developed analytical methods to compute surface strains from the measured displacements by digital image correlation (DIC). We showed that (1) the human sclera stiffens with age, and (2) the meridional strain response of glaucoma specimens is stiffer in the peripapillary sclera. After mechanical tests, we measured the collagen fiber structure of 11 normal and 10 glaucoma specimens using wide-angle x-ray scattering (WAXS). The degree of fiber alignment was lower in older sclera, and the spatial variations in the degree of fiber alignment around the ONH differed between normal and glaucoma specimens. We applied a specimen-specific inverse finite element method to calculate the material properties of these specimens from the inflation tests and the WAXS-measured collagen anisotropy. This method confirmed the age and glaucoma-related alterations to the biomechanical response of the sclera. It also suggested that the collagen fiber structure of the peripapillary sclera determines in important ways the deformation of the ONH. In contrast, the midposterior sclera has a structurally isotropic behavior.
We concluded that the glaucoma-related structural and biomechanical differences may represent scleral remodeling events in response to the disease, or baseline properties that contribute to axon damage.
