Effects of Scleral Stiffness on Biomechanics of the Optic Nerve Head in Glaucoma
Keywords: optic nerve head
biomechanics
glaucoma
stiffness

Glaucoma is a common cause of blindness worldwide, yet the etiology of the disease is unclear. A leading hypothesis is that elevated intraocular pressure (IOP) affects the biomechanical environment within the tissues of the optic nerve head (ONH), and that the altered biomechanical environment contributes to optic nerve damage and consequent loss of vision. The biomechanical environment of the ONH is strongly dependent on the biomechanical properties of sclera, particularly scleral stiffness. However there is significant variability in reported stiffness data for human sclera. Therefore, our research goal was to measure the stiffness of human sclera and incorporate this information into finite element models of the human eye to characterize and quantify the biomechanical environment within and around the optic nerve head region at different IOP levels. Human sclera adjacent to the optic nerve head showed highly nonlinear, nearly isotropic and heterogeneous stiffness which was found to be substantially lower than that previously assumed, particularly at lower levels of IOP. The products c*c1 and c*c2, measures of stiffness in the latitudinal and longitudinal directions from the Fung constitutive model, were 2.9 ± 2.0 MPa and 2.8 ± 1.9 MPa, respectively, and were not significantly different (two-sided t-test; p = 0.795). Scleral stiffness was not statistically different between left and right eyes of an individual (p = 0.952) and amongst the quadrants of an eye (p = 0.412 and p = 0.456 in latitudinal and longitudinal directions, respectively). Three stress-strain relationships consistent with the 5th, 50th and 95th percentiles of the measured scleral stiffness distribution were selected as representatives of compliant, median and stiff scleral properties and were implemented in a generic finite element model of the eye using a hyperelastic five-parameter Mooney-Rivlin material model. Models were solved for IOPs of 15, 25 and 50 mmHg. The magnitudes of strains at the optic nerve head region were substantial at even the lowest applied IOP (15 mmHg) and increased at elevated IOPs (e.g. the third principal strain in the compliant model reached as much as 5.25% in the lamina cribrosa at 15mmHg and 8.84% in the lamina cribrosa at 50 mmHg). Scleras that are “weak”, but still within the physiologic range, are predicted to lead to appreciably increased optic nerve head strains and could represent a risk factor for glaucomatous optic neuropathy. As IOP increased from 15 to 50 mmHg, principal strains in the model with a compliant sclera increased at a lower rate than in the model with a stiff sclera. We quantified the biomechanical environment within and around the optic nerve head region using a range of experimentally measured mechanical properties of sclera and at different IOPs. We showed that IOP-related strains within optic nerve head tissues can reach potentially biologically significant levels (capable of inducing a range of effects in glial cells) even at average levels of IOP and for typical human scleral biomechanical properties.