Pathogenesis of cervical spondylotic myelopathy.

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Abstract
OBJECTIVE: To determine whether either of two mechanical theories predicts the topographic pattern of neuropathology in cervical spondylotic myelopathy (CSM). The compression theory states that the spinal cord is compressed between a spondylotic bar anteriorly and the ligamenta flava posteriorly. The dentate tension theory states that the spinal cord is pulled laterally by the dentate ligaments, which are tensed by an anterior spondylotic bar. METHODS: The spinal cord cross section, at the level of a spondylotic bar, is modelled as a circular disc subject to forces applied at its circumference. These forces differ for the two theories. From the pattern of forces at the circumference the distribution of shear stresses in the interior of the disc-that is, over the transverse section of the spinal cord-is calculated. With the assumption that highly stressed areas are most subject to damage, the stress pattern predicted by each theory can be compared to the topographic neuropathology of CSM. RESULTS: The predicted stress pattern of the dentate tension theory corresponds to the reported neuropathology, whereas the predicted stress pattern of the compression theory does not. CONCLUSIONS: The results strongly favour the theory that CSM is caused by tensile stresses transmitted to the spinal cord from the dura via the dentate ligaments. A spondylotic bar can increase dentate tension by displacing the spinal cord dorsally, while the dural attachments of the dentate, anchored by the dural root sleeves and dural ligaments, are displaced less. The spondylotic bar may also increase dentate tension by interfering locally with dural stretch during neck flexion, the resultant increase in dural stress being transmitted to the spinal cord via the dentate ligaments. Flexion of the neck increases dural tension and should be avoided in the conservative treatment of CSM. Both anterior and posterior extradural surgical operations can diminish dentate tension, which may explain their usefulness in CSM. The generality of these results must be tempered by the simplifying assumptions required for the mathematical model.