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I. A. Sigal, H. Yang, M. D. Roberts, J. Grimm, C. F. Burgoyne, J. C. Downs; Lamina Cribrosa Displacement (LCD) and Scleral Canal Expansion (SCE) Due to Acute IOP Elevation. A Parametric Study Using Eye-Specific Models. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2129.
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Traditional thought has been that an acute increase in IOP causes substantial posterior LCD and near zero SCE. Recent experimental [Yang et al IOVS, 2009, 50(12)] and numerical [Sigal et al Exp Eye Res, 2007, 85(3)] evidence, however, suggests the opposite: that increased IOP deforms the peripapillary sclera, leading to sufficient SCE to pull the lamina taut, counteracting the direct effects of IOP on the lamina and producing small LCD. Our goal was to study how LCD and SCE relate to each other, and how they depend on the geometry and mechanical properties of the lamina cribrosa and sclera.
We reconstructed six eye-specific, baseline finite element models of the lamina and sclera of three monkeys. The models were parameterized using morphing techniques [Sigal et al J Biomech, 2009 Epub], which enabled us to produce 788 models related to the baselines with controlled variations in geometry and material properties. We varied 8 factors: scleral canal size and eccentricity, scleral thickness and modulus, laminar thickness, modulus, and position (curvature), and eye. A two-level factorial statistical analysis was used to determine factor influences and interactions on the LCD and SCE predicted for an IOP increase of 5 mmHg.
LCD depended mostly on lamina modulus and position. SCE depended mostly on sclera modulus and thickness. Both LCD and SCE were strongly influenced by interactions between the factors. Canal size and laminar thickness were less influential. SCE ranged from 2 to 16 µm. For 94% of factor combinations LCD was small (-10 to 10 µm), and an association between SCE and LCD was discernible. Only 6% of factor combinations resulted in substantial posterior LCD (>20 µm), and 25% of those also had large SCE.
The results suggest that the conceptual framework of a mechanically weak sclera pulling the lamina taut applies in many, but not all, factor combinations. Laminar properties also play an important role in its response to IOP. A small number of factor combinations resulted in peculiarly large LCD, which may indicate biomechanical sensitivity to elevated IOP.
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