A simple model is presented that explains the observed function and failure of the intervertebral disc in compression, torsion, and bending; this model is based upon the observed arrangement of collagenous fibers in the annulus fibrosus. The fibers are considered to have the same mechanical properties as tendon; thus the stresses required to produce a given deformation and which irreversibly damage the fibers can be predicted. Predictions of the mechanical behavior of the disc are in good agreement with published results for compression and torsion; no comparable experiments have been performed for bending. It is further predicted that torsion and bending are likely to cause annular failure and protrusion. Failure is likely to occur posteriorly because of the effect of forward bending and because in the flattened and reentrant discs of the lumbar spine, torsional stress is concentrated at the points of maximum curvature. The structure of the disc tends to protect the collagenous fibers in forward bending and torsion. Compression is predicted to cause end-plate fracture rather than annular failure.
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