[Segmental instability and lumbar spinal canal stenosis. Theoretical, clinical and surgical aspects].

Clinical, radiological, surgical and experimental observations make it possible to construct a rational theory of the segmental lumbar instability which explains the pathogenesis of spondylosis, of stenosis and of several cases of disc displacement as well. This theory helps us to choose the right surgical procedure. Spinal stenosis, degenerative spondylolisthesis and some cases of disc displacement are seen as part of a degenerative process with its starting point in regressive changes of the intervertebral disc and lateral joint. This leads to a marked instability of the affected mobile segment. Signs and symptoms of unstable lumbar spine are discussed. In such cases, if decompression of the roots by laminectomy, facetectomy or discectomy is performed without fusion, the consequences are usually treated but not the main cause of the trouble, namely the instability. Finally, we describe a procedure which we prefer as the safest and best of all, using selected decompression by means of joint screws (spondylodesis) as suggested by F. Magerl (41-44).