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Editors Selection IGR 9-4

Clinical examination methods: ONH-model

Comment by Crawford Downs on:

13014 Factors influencing optic nerve head biomechanics, Sigal IA; Flanagan JG; Ethier CR, Investigative Ophthalmology and Visual Science, 2005; 46: 4189-4199

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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. The dearth of available data is due to the technical challenges involved in the measurement of ONH tissue mechanical properties (stiffness) and the complexity of the ONH geometry.

Further complicating the study of ONH biomechanics is the biologic variability in the load-bearing structure, which includes geometry (scleral thickness, neural canal shape and size, laminar pore size and beam thickness, etc.), and tissue stiffness, which may change with age, pathology, extracellular matrix (ECM) composition, and connective tissue remodeling. Sigal et al. (798) have constructed an idealized parametric finite element model of the human eye, within which various geometric and tissue stiffness parameters can be varied to determine which of those parameters are most important in determining the response of the ONH to IOP. This paper is important in several ways. First, their results suggest that there are several factors that dominate the ONH's biomechanical response to IOP, many of which are not clinically intuitive. Within the model, the five factors identified as the most important determinants of ONH biomechanics are, in rank order: the stiffness of the sclera, the size of the eye, IOP, the stiffness of the lamina cribrosa, and the thickness of the sclera. As the authors acknowledge, these results should be viewed with some caution due to the simplifying assumptions necessary to construct the model.

In rank order: the stiffness of the sclera, the size of the eye, IOP, the stiffness of the lamina cribrosa, and the thickness of the sclera are identified as the most important five determinants for ONH biomechanics within the model

The most important limiting assumptions are the 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. All these remaining factors likely work with the considered parameters in complex ways to contribute to an individual ONH's susceptibility to IOP-related glaucomatous damage, and more work is needed to elucidate these mechanisms. This body of work represents a careful application of a powerful technique that has pushed computational biomechanical modeling of the posterior pole to a new level of sophistication.

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