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S. Kodiyalam, M. D. Roberts, J. Grimm, I. A. Sigal, R. T. Hart, C. F. Burgoyne, J. C. Downs; Beam-Level Stress and Strain in the Lamina Cribrosa of Normal and Early Glaucomatous Monkey Eyes. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3667.
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© ARVO (1962-2015); The Authors (2016-present)
To determine beam-level IOP-related stress and strain in the lamina cribrosa of normal and early glaucomatous (EG) monkey eyes.
We have developed parallel finite element code to handle large-scale, voxel-based models of 3-D reconstructions of the lamina cribrosa [IOVS, 2004; 45:4388]. Displacement boundary conditions for the laminar connective tissue (LCT) corresponding to expansion of the scleral canal are obtained from macro-scale continuum finite element models of the entire posterior pole. An IOP load of 35 mm Hg is applied uniformly to the anterior surface of neural tissue encapsulating the LCT, yielding configurations having up to 17 million voxel-based elements. We assigned Young's moduli of 180 MPa and 63 MPa to the normal and EG LCT, respectively, as determined by fitting the average posterior displacement of the LCT in the models to experimental displacement data.
In the normal eye, the hydrostatic pressure within the neural tissue becomes compressive everywhere, which is consistent with observations in the dog [IOVS 1995; 36:1163-72]. Overall, the mean tensile strain was 2.9% and 2.6% and the mean von Mises stress was 16 xIOP and 41 xIOP in the LCT of the EG and normal eyes, respectively. In the anterior half of the lamina in both eyes, stress and strain was highest in the central region and lowest in the periphery. In the posterior lamina, the opposite was observed (figure). On average, stress and strain were largest at the surface of the individual LCT beams and decreased with depth into the beams in both eyes.
In this monkey, the predicted mean von Mises stress is lower and mean tensile strain is higher in the LCT of the EG eye compared to its contralateral control. There are large variations in the biomechanical response of the lamina cribrosa at the regional and beam levels. These findings need to be confirmed in models of additional eyes from similarly treated monkeys.
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