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Kristin M. Myers, Frances Cone, Harry Quigley, Thao D. Nguyen; Modeling the Inflation Response of C57BL/6 Mouse Sclera. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6249.
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To measure the material properties for younger (2 month) and older (11 month) C57BL/6 mouse sclera.
An inverse finite element (FE) method was used to optimize parameters of a nonlinear, anisotropic, distributed fiber material model to fit previously reported scleral inflation data. Scleral geometry was modeled using a hyperbolic fit of the measured specimen material points at 6mmHg. The 2-D geometry was revolved 90°, with the optic nerve head (ONH) extending 100µm from the apex. The scleral material model consisted of a nearly incompressible isotropic Neo-Hookean matrix, shear modulus µs, and regions of transversely isotropic and anisotropic fibers. Fiber stress response was modeled using an exponential function with an initial fiber stiffness αo and a stiffening parameter β. Scleral regions of preferentially aligned circumferential fibers were defined within 100µm and 500µm of the ONH and the limbus, respectively, characterized by model parameters nONH and nlimbus. The ONH was modeled as a Neo-Hookean material, shear modulus 0.1•µs. Symmetric boundary conditions were used, with the limbal nodes fixed. FE calculations and an optimization scheme were performed using the software Tahoe and Dakota.
The figure reports the averaged material properties and representative best-fit model predictions. The limbal region showed possible fiber alignment. However, nONH was nearly zero. β and αo were larger for the older tissue, indicating age has a possible stiffening effect on fiber strength. However, this result was not significant.
The model reproduced well the displacement profile and the strain-stiffening behavior. nONH indicated a weak fiber alignment, however previous evidence has shown circumferential fibers near the ONH. This discrepancy arose because the peripapillary strains for this inflation set-up were too small to detect the anisotropy contribution. Current efforts include improving the inflation testing set-up and investigating scleral tissue from glaucoma models.
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