Biomechanics of lumbosacral dural sac. A study of flexion-extension myelography.

Measurements were performed on 40 lateral lumbar myelograms in flexion and extension with the object of analyzing changes in position and shape of the dural sac in spinal movements. There proved to be an anterior displacement of the entire lumbar dural sac in lumbar extension, most likely caused by shortening and thickening of the flaval ligaments. In addition, the anterior dural surface was indented at the L3-4 and L4-5 interspaces by posterior bulging of the discs in extension. This encroachment was partially compensated by dural bulging into areas with a rich and compressible venous plexus: behind the vertebral bodies and the L5-S1 disc. While the patterns of dural movements showed individual variations, these trends were found in all diagnostic and anatomic subgroups. One subgroup (with root involvement at L4-5) showed marked dorsal encroachment upon the dural sac in extension at the same level. The clinical implications of these findings are discussed.