Loss of elasticity in the aging human sclera

Purpose: To determine the age-related alterations in the elastic response of 30 posterior scleral shells from human donors, 27-90 years old. Methods: 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. Results: The scleral shear modulus 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). Conclusions: 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 crosslinking and loss of the elastin-driven recoil. The loss of elasticity should lead to larger high frequency IOP fluctuations in the elderly.