Finite element modeling of optic nerve head biomechanics.

PURPOSE: Biomechanical factors have been implicated in the development of glaucomatous optic neuropathy, particularly at the level of the lamina cribrosa. The goal of this study was to characterize the biomechanics of the optic nerve head using computer modeling techniques.
METHODS: Several models of the optic nerve head tissues (pre- and postlaminar neural tissue, lamina cribrosa, central retinal vessel, sclera, and pia mater) were constructed. Stresses, deformations, and strains were computed using finite element modeling for a range of normal and elevated intraocular pressures. Computed retinal surface deformations were compared with measured deformation patterns in enucleated human eyes. A sensitivity analysis was performed in which tissue properties and selected geometric features were varied.
RESULTS: Acute IOP-induced deformation of the vitreoretinal interface was highly dependent on optic cup shape but showed a characteristic "W-shaped" profile that did not match the deformation of the anterior surface of the lamina cribrosa. The central retinal vasculature had surprisingly little effect on optic nerve head biomechanics. At an IOP of 50 mm Hg, strains (fractional elongation) in the lamina cribrosa averaged 4% to 5.5%, dependent on model geometry, with maximum strains up to 7.7%. Strains in the lamina cribrosa were more dependent on scleral stiffness, scleral thickness, and scleral canal diameter than on lamina cribrosa stiffness and optic cup shape.
CONCLUSIONS: Computed levels of strain in the lamina cribrosa are biologically significant and capable of contributing to the development of glaucomatous optic neuropathy, even without considering the probable accentuating effect of the lamina cribrosa's microarchitecture. Depending on optic cup shape, IOP-induced deformation of the vitreoretinal interface may not match lamina cribrosa deformation. This finding implies that scanning laser tomography has limited ability to estimate lamina cribrosa deformation when imaging the anterior topography of the optic nerve head. Biomechanical effects in the lamina cribrosa depend strongly on scleral properties.