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C. R. Ethier, D. Zeng, A. T. Read, D. W. Chan, H. Gong, M. Johnson; Pressure-Induced Deformation of Schlemm’s Canal Endothelial Cells. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1633.
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Schlemm’s canal endothelial (SCE) cells are exposed to a basal-to-apical pressure difference that causes the cells to deform. We used finite element models and measured mechanical properties to determine the maximum pressure drop that SCE cells can support.
Finite element models of typical SCE cells were developed using scanning (SEM) and transmission (TEM) electron micrographs. Based on SEM images of the inner wall endothelium from 8 human eyes, 3 typical cellular "footprints" were chosen. Thickness profiles were measured from TEM images of 10 inner wall cells from 5 human eyes. Cell models were constructed using the 3 typical footprints and two possible thicknesses: (i) the average thickness of the cells; and (ii) the maximum thickness, the latter corresponding to a very "strong" cell (conservative case). Cells were assumed to be attached only at their periphery.A commercial finite element package ABAQUS was then used to compute: (i) cellular deformations for different pressure drops across these cells and; (ii) the maximum pressure drop that could be supported by the cells. A critical input to this analysis was the Young’s modulus of elasticity of the cells, a material parameter characterizing SCE stiffness. This parameter was measured experimentally, giving a mean value of 1120 Pa. We used this value in our simulations, and also a much larger value of 104 Pa, corresponding to the highest modulus measured for any endothelial cell type.
With a modulus of 1120 Pa, an inner wall cell of average thickness could maximally support a load of 1-1.5 mm Hg, while the strongest inner wall cells (with the maximum thickness) would support a load up to 2.8 mm Hg before failing. Using the conservative modulus of 104 Pa, we found that the strongest cells could support at most a load of 25 mm Hg.
Our results suggest that the pressure drop across the inner wall cannot be more than 2.8 mmHg. Even in the most conservative case of a very strong and thick SCE cell, the maximum pressure drop that could be supported was less than the IOP that can be experimentally achieved without apparent damage to the inner wall (30 mmHg or more). Thus, if there is any appreciable outflow resistance in the inner wall endothelium then SCE cell-substratum attachment is critical in controlling the biomechanics of the inner wall of Schlemm’s canal.
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