The Optic Nerve Head Pore Pressure Gradient in Glaucoma as Determined by Porohyperelastic Finite Element Modeling

Abstract

Purpose: : FEMs based upon elastic material laws, have previously been used to provide parametric studies of mechanical–structural ocular properties and to support models for ganglion cell damage associated with elevated intraocular pressure (IOP) and glaucoma. Porohyperelastic finite element models (PHE FEMs) of the eye are used to determine the optic nerve head pore pressure and pore pressure gradient in a glaucomatous eye. The pore pressure gradient may play a role in the development of glaucoma by directly impacting the optic nerve axons or by causing mechanical or microvascular damage.

Methods: : Porohyperelastic (PHE) FEMs were extended to quantify deformation, stress, and the tissue fluid pressure environment in the eye including at the lamina cribrosa (LC) and the axons in the optic nerve head (ONH). A 2–dimensional axisymmetric finite element model of the eye was created using ABAQUS. Ocular material properties were based on data in previous studies. An inlet flow at the ciliary processes, Q_CP = 2.69 µL/min was applied. Trabecular permeability, ktm = 5.4x10^–12 m²/Pa–sec ; retino–choroidal–scleral permeability, krcs = 8.15x10^–16 m²/Pa–sec, and the uveal–scleral outflow, Q_UV = 0.4 µL/min. The permeability of the lamina cribrosa was varied and the resultant pore pressure and pore pressure gradient were determined.

Results: : At an intraocular pressure of 50 mm Hg, variation of the k_LC from 1.6x10^–5 mm/sec to 1.6x10^–4 mm/sec increased the maximum pore pressure from 6.4 to 21.6 mm Hg. Variation of the k_LC also dramatically altered the pore pressure gradient in the retrolaminar optic nerve. Deformation and stress gradients were significantly impacted by the pore pressure gradient.

Conclusions: : A porohyperelastic FEM of a glaucomatous eye demonstrated the development of significant pore pressure gradient changes with variation of the laminar cribrosa permeability. The pore pressure gradient may play a significant role in glaucomatous optic nerve damage by directly affecting axons or by affecting mechanical or microvascular behavior. Porohyperelastic finite element models can be used to quantify the role of pore pressure or pore pressure gradients in glaucoma.

Keywords: optic disc • intraocular pressure • lamina cribrosa