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I.A. Sigal, J.G. Flanagan, C.R. Ethier; Identification of the Main Factors Influencing Optic Nerve Head Biomechanics . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1268.
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Purpose: The biomechanical environment within the optic nerve head (ONH) may play a role in retinal ganglion cell loss in glaucomatous optic neuropathy. Here we use bioengineering tools (finite element analysis) to determine the anatomical and biomechanical factors that most strongly influence the response of the ONH to changes in intraocular pressure (IOP). Methods: A computational model of the eye, including ONH tissues, was constructed as previously described (IOVS, 45:4378, 2004). We carried out a systematic parametric analysis in which each of 21 factors was varied independently. These factors included: the stiffness of the five tissue regions (sclera, pre and post–laminar neural tissue, pia mater and lamina cribrosa); the compressibility of the pre–laminar nerve tissue; the IOP; and 14 geometric factors, including thickness of the pre–laminar neural tissue, lamina cribrosa, pia mater and sclera at different points, the shapes of the scleral canal and cup and the eye globe radius and thickness. For each factor a range of admissible input values was defined, and commercial software (ANSYS v. 6.1) was used to compute the biomechanical response of ONH tissues, as quantified through a set of outcome measures. These were: the peak and mean stress and strain (% deformation) in the pre–laminar neural tissue and lamina cribrosa; the thickness of the pre–laminar neural tissue and lamina cribrosa at the axis of symmetry; and the diameter of the scleral canal. For each outcome measure, the influences of all input factors were summed to give a "total response", and then the influence of a single factor was quantified as a percentage of this total response ("relative response"). Results: For most outcome measures, the stiffness of the sclera was the most important input factor. The five input factors that had the largest relative responses when summed across all outcome measures were, in ranked order: stiffness of the sclera, radius of the eye, stiffness of the lamina cribrosa, IOP and stiffness of the retina. The five least important input factors, in reverse ranked order, were: retinal thickness, rate of enlargement of the retrobulbar optic nerve, thickness of the pia mater, thickness of the retina and rim height. Conclusions: The biomechanical properties of the sclera have a surprisingly large effect on ONH biomechanics. Individual–specific variations in scleral properties could therefore be an important risk factor for glaucomatous neuropathy. Eye size and lamina cribrosa biomechanical properties are other predicted risk factors.
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