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Editors Selection IGR 15-4

Basic Science: Scleral properties

Comment by Chris Leung on:

59507 Age- and Race-Related Differences in Human Scleral Material Properties, Grytz R; Fazio MA; Libertiaux V et al., Investigative Ophthalmology and Visual Science, 2014; 55: 8163-8172

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Investigation of scleral material properties has provided valuable insights into the risk factors and pathogenesis of glaucoma. With 40 European and 22 African donor eyes, Grytz and colleagues recorded three-dimensional displacements of posterior scleral shells during inflation tests; fitted parameters of scleral material properties, which included the crimp angle (a global microstructural measure that define collagen fibril crimping), the shear modulus, the elastic modulus and other parameters that define the local variation in the anisotropic collagen architecture into a computational model that matched the predicted displacements with the experimental deformation measurements; and evaluated the impact of age and race on the scleral material properties. The key findings are that the posterior sclera is less compliant in donors of African than European descent and that there is age-related reduction in posterior scleral compliance. The authors suggested these age- and race- related differences may be consequential to an increase in collagen cross-linking, and/or loss of the elastin-driven recoil (which were captured in the model by an increase in shear modulus and a decrease in collagen fibril crimp angle) in the aged and persons of African descent. Although age- and race- related differences in scleral material properties have been previously reported, this study is unique in incorporating parameters at multiple scales, from displacement measurements obtained from inflation tests (macro-level) to computational analysis of the collagen fibril crimp (microlevel), thereby providing an informative approach to study the effect of age and race on the overall scleral compliance. Of note, interpretation of the model requires careful examination of the assumptions. For example, that the stiffness and microstructural parameters are assumed to be constant throughout the sclera and that the crimp angle is not a direct measurement but based on an inverse computational analysis. Inflation tests were performed within 48 hours postmortem but the hydration status of the sclera (which is also related to age) during the measurement is unknown. More important, it remains unclear to what extent the overall scleral compliance impacts the biomechanical responses of the peripapillary sclera and the lamina cribrosa at different levels of intraocular pressure. Nevertheless, this study is an important addition to the literature and has opened up new opportunities to investigate the relationship between tissue material properties and optic nerve degeneration in glaucoma

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