Porohyperelastic-Transport Finite Element Models of the Eye Using ABAQUS

Glaucoma is related to damage to nerve ganglion cells in the optic nerve head (ONH) including the lamina cribrosa, (LC). This disease is associated with elevated intraocular pressure (IOP) and possibly reduced trabecular meshwork (TM) outflow. The ABAQUS program was used to develop axisymmetric porohyperelastic (PHE) pore fluid finite element models (FEMs) to determine deformations, stresses, tissue fluid pressures (p^f), and mobile fluid flux in the eye. These FEMs simulated aqueous pressure-fluid flow fields in the anterior chamber via the TM and posterior pressure-flow fields in the vitreous body (VIT) and ONH. Constant inlet flow at the ciliary processes (CP) was applied. The anterior chamber was modeled as a highly porous material containing large amounts of fluid whereas the VIT was modeled as a gel with mobile fluid. All ocular soft tissues were considered to be linear, isotropic PHE materials. Posterior transport was regulated by varying the permeability of the LC, retina, choroid, and sclera material layers. Two FEMs, i.e. IOP=15 mm Hg (normal) and IOP=44 mm Hg (glaucoma) were developed by varying the permeability of the TM. Deformations and tissue fluid pressures, fluid flux (relative fluid velocities), and stresses were determined and agree well with experimental data and other numerical model results. The displacement of the LC was 21–62 μm; the LC pressure gradient was 25–73 mm Hg/mm; and the posterior outflow ranged from 5%–15% of the inflow at the CP. The PHE material law can be extended to include nonlinear permeability effects and mobile species transport using a porohyperelastic-transport-swelling (PHETS) theory in future FEMs.