A Compressible, Transversely Isotropic, Hyperelastic Constitutive Model of the Guinea Pig Spinal Cord White Matter

Slow compression spinal cord injuries occur when the spinal canal narrows, the consequence of degenerative, infective, or oncologic legion growth, and exerts pressure throughout the spinal cord. Transverse tissue compression results in an amalgamation of mechanical insults at the cellular level [1]. However, the mechanism of cellular injury has yet to be elucidated. We have recently developed a hyperelastic, isotropic plane strain finite element model (FEM) of the guinea pig spinal cord white matter response to transverse compression based on force-deformation curves measured in vitro. The strongest correlation with in vitro axonal injury density was the combination of the in-plane shear stress with the in- and out-of-plane normal stresses quantified using the FEM [2]. However, we hypothesize that the guinea pig spinal cord white matter is a transversely isotropic material. Material anisotropy must be incorporated into the FEM to achieve enhanced model accuracy, specifically, the prediction of axial stresses within the spinal cord parenchyma during transverse tissue compression. Therefore, the objective of the present study was to propose a compressible, transversely isotropic, hyperelastic constitutive model of the guinea pig spinal cord white matter.