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M. J. Girard, J.-K. F. Suh, M. Bottlang, C. F. Burgoyne, J. C. Downs; Age-Related Alterations in the 3D, Nonlinear, Anisotropic Mechanical Properties of Non-Human Primate (NHP) Posterior Sclera. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4058.
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New custom experimental and computational tools were developed to investigate the age-related differences in 3D mechanical properties of posterior sclera from old (>18 years) and young (<2 years) NHP following acute elevation of intraocular pressure (IOP) from 5 to 45 mm Hg.
Immediately following enucleation the posterior scleral shell from both eyes of 8 NHP (4 young and 4 old) were individually mounted at the equator on a custom-built pressurization device. IOP was incrementally increased from 5 to 45 mm Hg, and the 3D displacement was measured using electronic speckle pattern interferometry. The geometry of each scleral shell was reconstructed from data generated by a 3D digitizer arm (shape) and a 20 MHz ultrasound probe (thickness). A fiber-reinforced constitutive model that includes both predominant orientation and stretch-induced stiffening of collagen fibers was applied to each reconstructed geometry, and a unique set of mechanical properties were fitted to the experimental displacements using a global optimization algorithm. Effective elastic modulus and structural stiffness, defined as the product of thickness and effective elastic modulus, were mapped for each shell.
The 3D experimental displacements matched the predictions of the model well. In all eyes, effective elastic modulus increased dramatically with IOP, which indicates that the sclera is mechanically nonlinear. Effective elastic modulus and structural stiffness were significantly higher for the old NHP group (p<0.001) for both the peripapillary and peripheral sclera regions (Figure). In all eyes, the peripapillary sclera exhibited the lowest effective elastic modulus, but its overall structural stiffness was higher than the thinner peripheral sclera.
Older NHP sclera is significantly stiffer than young NHP sclera and is therefore subject to higher stresses but lower strains at all levels of IOP. The contribution of these findings to age-related susceptibility to glaucomatous optic nerve head damage in humans remains to be determined.
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