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Ian A Sigal, Bingrui Wang, Bin Yang, Po-Yi Lee, Tian Yong Foong, Bryn Brazile, Yi Hua; Collagen fiber interweaving strongly influences sclera stiffness, and likely plays a central role in globe mechanics. Invest. Ophthalmol. Vis. Sci. 2019;60(9):6177.
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© ARVO (1962-2015); The Authors (2016-present)
Studies of sclera structure and biomechanics have focused on properties such as collagen density and orientation, neglecting fiber interweaving. We carried out combined experimental and numerical analyses to test the hypothesis that interweaving plays a central role in scleral biomechanics.
Posterior sclera of sheep, pig, goat, monkey and human were cryosectioned coronally, and imaged using polarized light microscopy (PLM). Images were analyzed for fiber interweaving properties (e.g. bundle crossing density and angle). Selected sections were mounted on a custom jig for PLM imaging while under quasi-static and dynamic controlled stretch. Using tracking we evaluated the stretch-induced fiber-scale deformations. Finite element models representing interwoven and non-interwoven fibers were developed and parameterized to test the role of interweaving on tissue stiffness.
Fiber interweaving was widespread in the sclera of all species, varying by region. Regions of radial and circumferential fibers were less interwoven than elsewhere (p<0.01). Direct visualization of the sclera microstructure while under stretch revealed unexpected fiber bundle rotations in interwoven regions. Models predicted much higher structural stiffness with interweaving. For example, for a volume fraction of 32%, interwoven fibrils were 60% stiffer than non-interwoven fibers. The stiffening effects of fiber interweaving increased quickly with volume fraction, and were further enhanced by splitting fibers into fibrils. Surprisingly, fiber bending stiffness had a minor effect on overall stiffness, but it did affect the location of stress concentrations.
Both experiments and simulations showed that fiber interweaving is pervasive, can enhance sclera structural stiffness, and likely plays a central role in overall eye mechanics. Two regions of the sclera can have identical fiber density and orientation, yet distinct structural stiffness and response to load. It is therefore critical to characterize fiber interweaving in the sclera.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
Experiments: PLM of sclera evidencing collagen fiber interweaving; traces illustrate complex nonlinear, anisotropic deformations under stretch. Simulations: 5% equi-biaxial stretch induced larger forces in interwoven fibers indicating higher stiffness. Stiffness increased with fiber volume fraction and by splitting fibers into fibrils
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