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C.R. Ethier, I.A. Sigal, J.G. Flanagan; Effect of Scleral Properties on Optic Nerve Head (ONH) Biomechanics . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2370.
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Purpose: We consider the possible role played by biomechanical forces in the development of glaucomatous optic neuropathy. Earlier modelling studies showed that the sclera has a large effect on ONH biomechanics. Here we further quantify these effects and suggest a model of how scleral biomechanics influences the mechanical insult experienced at the level of the lamina cribrosa (LC). Methods: We used a previously validated model of the eye (IOVS, 45:4378, 2004) and commercial finite element software (ANSYS v 6.1) to compute the biomechanical response of the ONH under different conditions. Based on reported values, we allowed scleral stiffness (Young’s modulus) to vary from 1 to 9 MPa. Based on histologic cross–sections of human ONH, we also allowed five factors defining scleral geometry to vary: canal opening diameter (1900 ± 380µm), scleral thickness at the canal wall (400 ± 80µm), scleral thickness away from the canal wall (globe wall thickness; 800 ± 160µm), globe radius (12 ± 2.4mm), and peripapillary sclera geometry (almost no thinning to extended thinning up to 15° from the cup centre). For each configuration we computed peak and mean stresses and strains (% deformation) in 5 tissue regions – pre and post–laminar neural tissues, peripapillary sclera, LC and pia mater; as well as several measures of ONH deformation, such as changes in pre–laminar neural tissue, LC thickness and diameter of the scleral canal. Results: Changing scleral stiffness had a profound effect on the magnitude and location of the peak levels of strain within neural tissues and within the LC. Further, this dependence was non–linear and non–monotonic. For example, when scleral stiffness was 1 MPa, peak tissue strains at an IOP of 50 mmHg reached biologically significant values of 15.9% and 12.4% within the LC and pre–laminar neural tissue, respectively. When scleral stiffness was 9 MPa, the corresponding strains were only 3.8% and 3.9%. The effect of changes in sclera geometry were less strong, but still of a magnitude comparable to that of a 10 mmHg increase in IOP. Conclusions: Scleral biomechanical properties are a major determinant of ONH biomechanics. This initially counter–intuitive result can be explained as follows: the sclera is the main load–bearing tissue in the eye, and therefore undergoes deformations that are directly transmitted to optic nerve head tissues through mechanical coupling, i.e. acute deformation of ONH tissues depends critically on the indirect effects of IOP on the sclera. This implies that scleral mechanical properties in normal and glaucomatous eyes need to be more carefully studied.
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