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M. Girard, J. C. Downs, C. F. Burgoyne, M. Bottlang, J.-K. F. Suh; Anisotropic and Nonlinear Mechanical Behavior of Monkey Posterior Sclera Under Intraocular Pressure. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3304.
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© ARVO (1962-2015); The Authors (2016-present)
To characterize material properties of monkey posterior sclera following acute elevations of intraocular pressure (IOP).
The posterior scleral shell of a rhesus monkey eye was mounted at the equator on a custom-built pressurization device, which applied IOP with isotonic saline. IOP was incrementally increased from 5 to 45 mmHg, and the 3-D displacement field of the sclera was measured at equilibrium using electronic speckle pattern interferometry. The outer surface geometry of the shell, sampled with a 3-D digitizer arm, and the scleral thickness, measured with a 20 MHz ultrasound transducer, were combined to produce a finite element (FE) mesh. A fiber-reinforced constitutive model incorporating collagen fiber nonlinearity and a probability distribution for collagen fiber orientation was applied to the FE mesh. Surface node displacements were curve-fitted to the experimental displacement data using a genetic optimization algorithm, which estimated a unique set of material parameters (c1: matrix modulus, c3: fiber modulus, c4: uncrimping rate of fibers, k: fiber concentration factor, θ: principal fiber orientation).
Experimental displacement field was nonlinear and well matched by the proposed model. The matrix modulus was 0.35 MPa and the initial fiber modulus was 3.9 MPa, which is consistent with fiber moduli reported in previous studies. Sclera was thickest in the inferior temporal region adjacent to the optic nerve head. The model predicted predominant collagen fiber orientation to be circumferential in the peripapillary sclera region, which matches reported histologic results (figure).
Posterior scleral deformation following acute IOP elevations appears to be non-linear and governed by the underlying scleral collagen microstructure as predicted by FE modeling. This preliminary study is the first 3-D modeling of scleral mechanics. The method will now be used to characterize scleral mechanics in normal (young and old), early and moderately glaucomatous monkey eyes.
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