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S. Kodiyalam, M. D. Roberts, I. A. Sigal, R. T. Hart, C. F. Burgoyne, J. C. Downs; Relative Contributions of Direct IOP and Scleral Canal Expansion on the Biomechanics of the Lamina Cribrosa (LC) of a Normal Monkey Eye. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4893.
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To test the hypothesis that in a normal monkey eye, IOP determines the magnitude and distribution of stress and strain in the LC and influences LC thickness through two mechanisms: 1) direct action of IOP on the anterior LC surface, and 2) IOP-induced scleral canal expansion.
A large-scale, voxel-based, finite element (FE) model of the LC connective tissues from a normal monkey eye was constructed from a 3D ONH reconstruction [IOVS, 2004; 45:4388]. A pressure load (ΔIOP) corresponding to an IOP increase from 10 to 45 mmHg was applied directly to the anterior surface of elastically weak neural tissue encapsulating the LC microarchitecture. In addition, an IOP-induced scleral canal expansion (SCE) was applied to the LC at its scleral canal wall insertion, as obtained from a macro-scale continuum FE model of the entire posterior pole of the same eye pressurized from 10 to 45 mmHg. The LC was assigned a Young’s modulus of 3.24 MPa by fitting the average posterior displacement of the LC in the models to experimental data [IOVS, 2003; 44:623]. Von Mises stress and maximum principal strain were calculated regionally and by depth through the thickness of the LC.
Regional mean values of LC stress and strain are strongly correlated (Figure). Generally, ΔIOP alone causes stress and strain to increase with depth into the LC centrally, but decrease peripherally. Generally, SCE alone induces the opposite behavior. The combined effects of ΔIOP and SCE induced maximum stress and strain midway through the thickness of the LC in both the central and peripheral regions, and caused LC thinning by 2.8% centrally and 5.5% peripherally.
The direct action of IOP on the ONH surface (ΔIOP) and IOP-induced SCE influence stress and strain patterns in the LC differently, but when combined result in maximum stress and strain occurring midway through the depth of the LC. The combined loadings thinned the LC, especially in the periphery.
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