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Editors Selection IGR 11-2

Basic Research - Sclera: Parametric finite element model

Comment by Crawford Downs on:

46445 An applet to estimate the IOP-induced stress and strain within the optic nerve head, Sigal IA, Investigative Ophthalmology and Visual Science, 2011; 52: 5497-5506

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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. Sigal (1654) has further exercised a previously reported idealized parametric finite element model of the human eye, within which various geometric and tissue stiffness parameters were varied. The results were fit to a polynomial and encapsulated into an easy-to-use, publicly available applet. The applet is an important contribution to the community because it allows the non-engineer to estimate the biomechanical behavior of a generic ONH under various user-specified conditions such as eye size, laminar stiffness, and scleral thickness and stiffness. The results of the various parameter manipulations are presented for IOPs of 5-15 mmHg in terms of quantitative estimates of median and peak von Mises stress, tensile strain, and compressive strain in the laminar and prelaminar neural tissues. As the author acknowledges, the applet's estimates should be viewed with caution due to the simplifying assumptions employed in the underlying finite element models, such as lack of consideration for regional laminar density and predominant beam orientation (both affect stiffness), the simplicity of the laminar and scleral geometries (limited to circular scleral canals, and uniform laminar and scleral thickness), and the simplicity of the assumed material properties (linear, elastic, and homogeneous). These assumptions are reasonable for computational studies of this type, but are non-physiologic and result in erroneous estimates of stress and strain. Hence, while the general trends of stress and strain output by the applet, i.e., increasing or decreasing, are likely to hold for most ONHs under the conditions input by the user, the numeric estimates of stress and strain cannot be trusted. Although the applet is unsuitable for determining the ONH biomechanics in individual patients (either naive or in response to a perturbation such as peripapillary scleral cross-linking), it can be used as a qualitative teaching tool that elucidates the general trends in ONH stress and strain when various parameters are manipulated.

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