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Editors Selection IGR 8-4

Basic Research: ONH Biomechanics

Comment by Crawford Downs & Philippe Denis on:

26774 Effects of scleral stiffness properties on optic nerve head biomechanics, Eilaghi A; Flanagan JG; Simmons CA et al., Annals of Biomedical Engineering, 2010; 38: 1586-1592

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Optic nerve head (ONH) biomechanics likely plays an important role in the development and progression of glaucoma, but it is not well understood. Eilaghi (1047) has further exercised a previously reported idealized axisymmetric finite element model of the human eye, within which nonlinear scleral stiffness parameters were assigned compliant, median or stiff values in accordance to previously published experimental values. In the present study, Eilaghi is the first to use experimentally derived nonlinear scleral material properties to determine how important the nonlinearity (the tendency for soft tissues to stiffen as they stretch) and range of stiffness are in determining the response of the ONH to IOP. This paper is incremental, but important because his results suggest that the ONH's biomechanical response to IOP is dependent not only on the basal stiffness of the sclera, but also on nonlinearity. The models with a compliant sclera predict that the strains in the lamina cribrosa are much higher than those in models with median or stiff sclera, which could increase glaucomatous susceptibility for those patients with severe myopia or other conditions that are coincident with more compliant sclera. As the authors acknowledge, these results should be viewed with some caution due to the simplifying assumptions employed, such as model's inability to consider regional laminar density or stiffness, the simplicity of the laminar and scleral geometries, and its limitation to scleral canals of circular shape. This body of work is the first to show that it is not only the scleral stiffness, but also its nonlinear response to stretch that influence the ONH biomechanical response.

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