Osteoporosis is a growing health care problem in the United States, with an associated annual cost of billions of dollars to society. Bone fracture is the overriding consequence of osteoporosis with the single highest occurrence is found in the vertebrae, estimated to be at hundreds of thousands cases annually. To minimize the incidence of these injuries through safety interventions or therapeutic regimens, a detail understanding of the fracture mechanisms and the effects of the varying vertebral properties on such occurrences is necessary. The finite element method has proven to be the most versatile means to study the detailed stress and strain distribution of any object under various loading conditions.

A detailed finite element (FE) model of the human thoracolumbar spinal column, consisted of vertebral level T11 through L2, was developed using anatomically accurate geometry from cadaveric measurements. Besides the inclusion of 4 vertebrae, this model incorporated such features as using transversely isotropic formulation to model the vertebral centrum material properties, simulating the intervertebral disc annulus by fiber-reinforced ground substance, and representing the anterior longitudinal ligaments with elastic elements. Furthermore, the elastic properties of the posterior joints, hypothesized to have significant influences on the vertebral responses, were determined by correlating the model’s predictions to the data recorded in the compressive testing of two spinal segments and a thoracolumbar spinal specimen of similar configuration.

The predictions of the FE model showed that the engaging mechanisms of the posterior joints, together with the mechanical behaviors of the intervertebral discs, caused the nonlinearity seen in the overall deflection of the thoracolumbar spinal column under comressive loading. Furthermore, the characteristics of the measured nonlinear load strain curves were primarily influenced by the engagement of the posterior joints. The finite element model was used to parametrically investigate the effects of some age-related changes (centrum modulus, thickness of the cortical shell and endplates, and cortical shell curvature) on the vertebral responses. Among the parameters investigated, the reductions in the centrum modulus produced most significant changes in the vertebral responses.