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Editors Selection IGR 12-3

Intraocular Pressure: ONH response to IOP

Comment by Crawford Downs on:

23892 Interactions between geometry and mechanical properties on the optic nerve head, Sigal IA, Investigative Ophthalmology and Visual Science, 2009; 50: 2785-2795

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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. Ian Sigal has further exercised a previously reported idealized parametric finite element model of the human eye, within which various geometric and tissue stiffness parameters can be varied. In the present study, Sigal (569) used a factorial analysis based on a Design of Experiments approach to determine how important the interactions between the varied factors are in determining the response of the ONH to IOP. This paper is important because his results suggest that, while several factors dominate the ONH's biomechanical response to IOP as suggested by his previous work, the factors' influences are interactive and interdependent. The most important determinants of ONH biomechanics remain the stiffness of the sclera and the lamina cribrosa, as previously reported, but their interactions were equally interesting. Increases in either scleral or laminar stiffness led to significantly lower average strains in the lamina, but the strains could still be high if either the sclera or lamina was stiff depending on the relative values of other factors.

The most important determinants of ONH biomechanics remain the stiffness of the sclera and the lamina cribrosa

Although it is unintuitive, in some cases laminar strains could be low when the sclera, the lamina, or both were compliant. 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 independent features of an individual eye, but also their particular combination that determine the ONH biomechanical response. Hence, while it will be helpful to clinically characterize one or two of the most important factors as biomarkers of ONH biomechanics, it is their relative values and the resulting interactions with other factors that will ultimately determine the eye's response to IOP.

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