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J. P. Vande Geest, B. R. Simon, R. I. Park, P. H. Rigby; Comparison of Poroelastic and Elastic Finite Element Models of Normal and Glaucomatous Conditions in the Eye. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3303.
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© ARVO (1962-2015); The Authors (2016-present)
Recent research using experimental models and elastic finite element models (FEMs) have suggested that the determination of biomechanical properties in ocular tissues may be important in the detection and development of glaucoma (Burgoyne et al. 2005, Sigal et al., 2005). We have developed porohyperelastic (PHE) FEMs of the human eye including the calculation of tissue fluid pressure (Pf) levels in the optic nerve head (ONH) (Park et al., ARVO 2006, Simon et al., 2003, 2005). The purpose of this study was to compare and quantify the differences in the biomechanical environment around the optic nerve head (ONH) using FEM simulations based on purely elastic versus PHE material models. The accurate determination of mechanical factors is essential in current and future models for ganglion cell damage associated with elevated intraocular pressure (IOP) and glaucoma.
FEMs were developed using geometry, boundary conditions, and material properties based on previous models (Simon ARVO 2006, Sigal et al., 2005). PHE and elastic simulations were performed on otherwise identical FEMs for ranges of IOP from 10 to 40 mmHg. The effective stress principle was used in the PHE simulation to separate the contributions of the HE solid and mobile fluid response. Parameters compared in the elastic and PHE FEM simulations included peak maximum principal strain, von Mises stress, Pf, and the gradient in Pf, i.e. dPf/dx. Specific attention was given to these fields in the ONH including the lamina cribosa (LC) and retrolaminar neural structures.
At an IOP of 10 mm Hg, the peak maximum principle strains in the LC were 0.003 and .01 for the PHE and elastic FEMs, respectively. The von Mises stresses in the LC were significantly lower in the PHE model compared to the elastic model; i.e. 0.8 versus 9.2 mmHg, respectively. Pore pressure gradients were largest across the LC (vitreous - LC - retrolaminar tissue) and LC displacements were different between PHE and elastic simulations.
The complex biomechanics in the ONH are important in the development of glaucoma. Elastic FEMs have previously given estimates of stresses and strains. Given the relatively high fluid content in soft ocular tissue, a PHE model provides initial values for Pf and dPf/dx in and around the ganglia in the LC and ONH. The PHE models may also give better values for LC displacement that may be associated with the progression of glaucoma. Such information can provide new insights regarding the loss of ganglion cell function associated with this disease.
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