Abstract
Abstract: :
Purpose: Glaucomatous damage to the optic nerve is often related to elevated intraocular pressure (IOP) and possibly to reduced trabecular meshwork (TM) outflow. The lamina cribrosa (LC) is a potential damage site for the ganglion cell neurons. Deformation, tissue fluid pressure, and mechanical properties of connective tissues may affect ganglion cells according to mechanical and microvascular theories for glaucoma. Porohyperelastic (PHE) finite element models (FEMs) are used to describe fluid flow and structural response in the eye simulated in various configurations. Methods: ABAQUS axisymmetric PHE FEMs are developed to determine aqueous pressure–flows and deformations, stresses, and tissue fluid pressures in the eye including the TM and optic nerve head. Pressure or inlet flow (QCP) at the ciliary processes (CP) was applied. All tissues are PHE materials, e.g. the vitreous body (VIT) is a gel with mobile fluid and the anterior chamber a highly porous material containing large amounts of fluid. TM and LC permeabilities were prescribed. Posterior transport from the VIT was regulated by varying LC, retinal, choroid, and scleral permeabilities. Initial FEMs use linear materials; e.g. elastic modulus, ETM = 100, ELC = 5500 Kpa; Poisson ratio, ν = 0.45; and permeabilities, kTM = 1.93x10–6 and kLC = 1.12x10–6 mm/s. Results: Using these material/structural properties, FEM results for QCP = 2.69 µL/min (previously reported [1]) demonstrated a resultant IOP = 15 mm Hg and TM outflow flux, QTM = 2.32 µL/min. The overall deformations and tissue fluid pressures, mobile fluid flux (relative velocities), and stresses were determined. Relative fluid velocity fields in the anterior chamber were comparable to previously reported numerical model results [1]. The LC displacement (disk cup region) was 5.2 – 5.7 µm and the pressure gradient, 25 mm Hg/mm (vs. 30.8 for IOP = 17 mm Hg [2]). The LC outflow was 0.37 µL/min. The effects of decreased tissue permeability associated with local deformations [3,4] are simulated in the TM, LC, and posterior ocular structures by introducing deformation–dependent permeabilities in the FEMs. Conclusions: The PHE FEMs developed for this project simulate structural and fluid dynamic characteristics of the human eye. The FEM results reported in our models agree with available data and provide a benchmark for future models. The PHE material law can be extended to include mobile species transport using a porohyperelastic–transport–swelling (PHETS) model [4] for quantitative studies of drug transport strategies. References: [1] Heys et al, 2001; [2] Morgan et al, 2002; [3] Meroi & Schefler, 2003; [4] Simon et al, 2001.
Keywords: computational modeling • trabecular meshwork • intraocular pressure