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J Downs, F Suh, RT Hart, CF Burgoyne; Finite Element Model Validation of Viscoelastic Scleral Material Properties of Normal and Glaucomatous Monkey Eyes . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4042.
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Purpose: To determine the validity of the phenomenological representation of experimentally-derived viscoelastic scleral material properties for finite element analyses (FEA) of the scleral shell. In addition, to determine if the differences in the viscoelastic scleral material properties between normal and glaucomatous monkey eyes observed experimentally can be modeled computationally with FEA. Methods: The development of finite element models of the posterior pole and optic nerve head is currently underway to study the role of intraocular pressure (IOP) in glaucoma. To determine the validity of this type of computational modeling, we constructed two models of a scleral tensile testing specimen with geometries identical to that of our experimental peripapillary scleral tensile specimens. One model was assigned the viscoelastic material properties measured within sclera from a normal eye and the other model was assigned properties derived from an early experimental glaucoma eye. Each model was then subjected to identical, time-dependent loading consisting of a preload, followed by the application of extensometer arm contact forces, elastic cycles, a 6 min recovery period, and a stress relaxation test. This computational loading was identical in magnitude and time course to that applied experimentally during the uniaxial scleral tensile testing. We compared the stress (force/cross-sectional area) and strain (local deformation) responses of the experimental and computational results both within and across treatment groups. Results: 1. The FEA of the scleral tensile specimens accurately predicted the stress and strain responses measured in the experimental tensile tests for each set of material properties (normal and glaucomatous). 2. The analyses predicted the creep (time-dependent deformation under a constant load) observed during the experimental tensile testing of scleral specimens. 3. Differences in viscoelastic scleral material properties between normal and glaucomatous monkey eyes derived experimentally were accurately represented in the computational models. Conclusion: The finite element method and the phenomenological representation of experimentally-derived viscoelastic scleral material properties are valid for modeling IOP-induced stress and strain in the scleral shell. In addition, finite element models accurately predict the different stress and strain distributions resulting from differences in the viscoelastic material properties of peripapillary sclera from normal and early glaucomatous monkey eyes.
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