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B.R. Simon, R.I. Park, P.H. Rigby; Porohyperelastic Finite Element Models of Fluid Pressure and Flows in the Eye . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1297.
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Purpose: Finite element models (FEMs), described previously ; determined structural and fluid fields in the eye and demonstrated relative outflow via the trabecular meshwork (TM) and posterior vitreo–retinal–choroidal (VRC) pathways. This study extends those FEMs giving specific attention to TM outflow (QTM); uveoscleral flow (QUS); and to the VRC flow (QVRC) associated with the retina, choroid, sclera, and lamina cribrosa (LC) in normal and glaucomatous conditions. Mechanical and microvascular glaucoma theories suggest that ganglion cells in the optic nerve head (ONH) may be damaged when intraocular pressure (IOP) is elevated. Methods: Porohyperelastic (PHE) axisymmetric FEMs of the human eye were developed based on the geometry and mechanical properties reported in ). An inlet flow at the ciliary processes, QCP = 2.69 µL/min  was applied and outlet flows and/or resistance (permeability) were varied or prescribed, e.g. QVRC ranged from 0.01QCP  to 0.21QCP  for constant QUS = 0.4 µL/min  and normal IOP. A parametric FEM study with variable permeabilities of the TM (kTM) and the retina, choroid, sclera (kRS) was carried out to predict the associated changes in deformation; stress; tissue fluid relative velocity fields (vfr), pressure (pf), and pressure gradient (dpf/dx) for elevated IOP. Results: Representative steady state FEM results include IOP = 15 mm Hg, QVRC = 0.05QCP; maximum LC displacement, dLC = 21 µm; dpf/dx–LC = 25 mm Hg/mm for kTM = 2.6x10–14 m2 /Pa–s and kRS = 6x10–17 m2 /Pa–s. The gradient dpf/dx–LC was reported as 31 mm Hg/mm at IOP = 17 mm Hg . When kTM was decreased to 0.4x10–14 m2 /Pa–s, IOP increased to 44 mm Hg, dLC to 62 µm and dpf/dx–LC to 73 mm Hg/mm. Tissue stresses and pf were also increased in the ONH at this elevated IOP. When kTM = 2.2x10–14 m2 /Pa–s and kRS was increased to 24x10–17 m2 /Pa–s, then QVRC = 0.19QCP for IOP = 15 mm Hg. Conclusions:PHE FEM results agree with available data and provide details of the deformation and stresses. The role of these structural parameters and additionally that of tissue fluid pressure and pressure gradients in the ONH are demonstrated. Such quantitative data may be used to evaluate damage to ganglion cells due to elevated IOP. Experimental and three–dimensional FEMs are under development that will allow more accurate predictions of (coupled) structural response and fluid – mobile species transport as well as drug transport processes in the human eye. References:  Simon et al, 2004;  Kaufmann, 1989;  Ariaie, et al, 1991;  Fatt, 1970;  Morgan et al, 2002.
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