Functional pathology of lumbar spinal stenosis.

This paper deals with the effect of motion upon the stenotic lumbar spinal canal and its contents. A review is presented of personal investigations and relevant data from the literature. The normal spinal canal and its lateral recesses are naturally narrowed by retroflexion and/or axial loading, as shown by anatomical, myelographic and computerized tomographic studies. Due to enough free play and protection by safety cushions, cauda equina and exiting nerve roots are not endangered. In stenosis this protection is lost, causing clinical symptoms of nervous compression of a posture-dependent character. Stenosis of the central portion of the spinal canal and of the lateral recess are dealt with separately. The more stenotic narrowing progresses, the greater the narrowing effect of retroflexion will become. This 'rule of progressive narrowing' explains why in severe stenosis even small motions will have a marked functional effect. The author contests the view of some investigators that vertebral 'instability' is essential to explain posture-dependent nerve root compression. Relative narrowing of the spinal canal is shown to predispose to manifestation of lumbosacral nerve root compression by other causes, for example disc herniation. Relative roominess of the canal is thought to explain why abnormal myelograms, suggestive of stenosis or disc herniation, occur in a relatively large percentage of asymptomatic individuals.