A poroelastic model of the optic nerve shows a significant effect of fluid pressure on the nerve fibers

Abstract

The structure of the optic nerve resembles a cylindrical composite where the pia mater surrounds the nervous tissue which is saturated with interstitial fluid. This interstitial fluid is necessary for effective nerve conduction of visual signals. The reaction of the optic nerve to physiological loads remains unknown. Current computational and material models do not fully capture the complexities of this tissue's structure, particularly the biofluid has not yet been considered as a load-supporting material. We developed a microstructurally motivated analytical model of a cylindrical composite with a poroelastic core and an elastic outer layer subjected to an axial load. We examined the effect of the geometry and the material parameters of the composite on the stress distribution across the composite. We found physiologically relevant conditions when the outer layer and the biofluid support most of the applied stress relative to the solid constituents of the core. The model shows that the fluid pressure can be as large as one third of the applied stress. The model makes possible the fluid pressure injuring nerve fibers. This scenario is missing in studies modeling the optic nerve as an elastic solid. We examined how variations in outer layer thickness and compressibility of animal nerves or materials stiffen the stress-strain response. This study provides guidelines for measuring and comparing the material parameters between diseased, aged, and healthy nerves and similar biomaterials. The model can be used to analyze mechanics of similar composites.