Acute low-back pain and the associated disability represent a national health problem of crisis proportions. While the etiology of back pain is varied, in many patients it is clinically associated with derangement of the intervertebral disc and disruption of the annulus fibrosus (a key structural tissue of the disc). Yet, the mechanisms by which spinal loading leads to unfavorable mechanical stress and injury remains unclear, particularly when associated with minimal trauma. A critical engineering tool for the study of disc injury is finite element modeling (FEM), which is currently the only means to obtain detailed predictions of disc stress produced by physiologic spinal loading. However, recent data from the applicant laboratory suggests that current FEM's may misrepresent stress in the annulus by several orders of magnitude. This discrepancy results from the use of annular material properties that are incompatible with recent experiments, and clearly limits the extent by which current models may be used to uncover relevant injury mechanisms. The annulus demonstrates three important biomechanical behaviors: 1) strong directional dependence (orthotropy); 2) non-linearity under large deformations (hyperelasticity); and 3) viscoelastic effects (poroelasticity). As a first step toward improved models of the intact intervertebral disc, the goal of this project is to develop a highly validated finite element representation of the annulus fibrosus which appropriately includes these biomechanical characteristics. Toward this end, the applicants propose to first derive an orthotropic, hyperelastic material law for the solid phase of the annulus. The hyperelastic formulation will be based on a continuum theory of fiber-reinforced composites and will be specified through simultaneous nonlinear regression to datasets from five separate experiments. Next, they will implement this material law using the user-defined-material feature of the commercially available FEM software ABAQUS. The hyperelastic solid behavior will be joined with the existing porous media capabilities to result in an orthotropic, poro-hyperelastic model of the annulus fibrosus. Finally, as a check on model performance, they will compare the FEM predictions of annular stress and pore fluid pressure to a sixth set of experimental data for the annulus tested at various rates in tension and compression under large deformations. They suggest that the outcome of their studies will be highly validated material and finite element models for the annulus fibrosus. In the future, they plan to extend this model to the full intervertebral disc, in attempts to develop better understanding of the mechanisms of acute back pain and degenerative disc disease.
