Intervertebral disc degeneration is a debilitating disorder of the spine, but the mechanisms involved in its initiation and progression are largely unknown. Certain compressive loading regimens have been shown to induce apoptosis and irreversible disc degeneration, while others do not. Some theories postulate that the effects of loading are dependent on time-history stress (or strain). In support of this, studies have shown that there can be benefits associated with specific levels of mechanical stimulation in musculoskeletal tissues. Based on preliminary and published data, we hypothesize the existence of a metabolic zone within which disc degeneration is inhibited. We expect that the borders of this zone will be defined by the hydrostatic and deviatoric stress histories of the annulus fibrosus and nucleus pulposus, respectively. Furthermore, we hypothesize that certain regimens of dynamic spinal bending motions can maintain a favorable time-history mechanical stress. Parametric analyses using a poro-hyperelastic finite element model of the disc will be performed to predict stress histories in order to define a set of candidate bending regimens for preventing disc degeneration. These regimens will then be tested experimentally to determine the effects on biosynthesis and apoptosis.
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