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Rafael Grytz, Massimo A. Fazio, Michael J. Girard, Vincent Libertiaux, Luigi Bruno, Stuart Gardiner, Christopher A. Girkin, J. Crawford Downs; Loss of Elasticity in the Aging Human Sclera. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2800.
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To determine the age-related alterations in the elastic response of 30 posterior scleral shells from human donors, 27-90 years old.
Posterior scleral shells were subjected to IOP elevations from 5 to 45 mmHg and the resulting full-field displacements were recorded using laser speckle interferometry. Eye-specific finite element models were generated based on experimentally measured scleral shell surface geometry and thickness. The elastic response of each scleral shell was fitted using a microstructure-based constitutive formulation, incorporating the anisotropic architecture and crimp form of scleral collagen fibrils (JMBBM 2(5):522-533, 2009). Inverse numerical analyses were performed to identify the intrinsic material and micro-structural parameters for each eye by matching model deformation predictions to experimental measurements (IOVS 50(11):5226-5237, 2009). Generalized Estimating Equation models were constructed to determine whether there was a significant effect of age on the fitted material and microstructural parameters while accounting for intra-donor correlations. The biomechanical effects of aging were predicted for a 27-year-old donor eye using the statistically derived parameters.
The scleral shear modulus (the stiffness of the ground substance) increased significantly and the collagen fibril crimp angle decreased significantly with age (p<0.001). The overall IOP-dependent collagen network strain also decreased with age in the modelled example eye (Figure).
The age-related loss of scleral elasticity is due to both a stiffer ground substance and the decreasing stretch at which the collagen fibrils uncrimp and stiffen. These changes may be due to increased collagen cross-linking and loss of the elastin-driven recoil. The loss of elasticity should lead to larger high frequency IOP fluctuations (e.g. ocular pulse amplitude) in the elderly.
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