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R. Grytz, M. J. Girard, M. Fazio, C. F. Burgoyne, J. C. Downs; Biomechanical Implications of Age-Related Alterations in Scleral Collagen Fibril Crimp. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2133.
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To investigate the age-related alteration in the microstructural crimp (waviness) of collagen fibrils and its impact on the IOP-related deformation response of posterior scleral shells from old (22.9 ± 5.3 years) and young (1.4 ± 0.8 years) rhesus monkeys.
A microstructure-based constitutive model incorporating collagen fibril crimp (JMBBM 2(5):522-533, 2009) was combined with a novel numerical algorithm to calculate the pre-existing stretch and stress environment in scleral shells loaded to 5 mmHg IOP. The pre-existing stress and stretch in the scleral shell of each eye were then used as the initial conditions in an inverse finite element analysis, which enabled the identification of the intrinsic microstructural and constitutive parameters of the tissue. This was accomplished by matching model deformation predictions to experimental inflation data for IOPs from 5 to 45 mmHg (IOVS 50(11):5226-5237, 2009). The fitted model parameters have clear physical interpretations based on the crimp form and the stiffness of collagen fibrils, and the stiffness of the ground substance.
The simulated deformations of each posterior scleral shell were in good agreement with those obtained experimentally. At 0 mmHg IOP, the collagen fibril crimp angle was significantly lower in old monkeys compared to young monkeys (p = 0.011). The elastic moduli of both the ground substance (responsible for scleral stiffness at low IOP) and of the collagen network (responsible for scleral stiffness at high IOP) were significantly higher in old monkeys (p < 0.01). At all IOP levels, scleral stretch was significantly lower in old monkeys (p < 0.001).
The age-related reduction in the elastic deformability of sclera from old monkeys is due to both stiffer non-fibrillar tissue constituents, and the lower stretch at which the collagen fibrils uncrimp and stiffen. The computed reduction in the scleral collagen fibril crimp during aging is consistent with experimental observations in rat tail tendons (Proc R Soc Lond B 180:293-315, 1972). These microstructural changes could play a role in the eye’s response to acute IOP fluctuations and potentially contribute to age-related susceptibility to glaucoma.
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