Instability in lumbar spondylolisthesis: a radiologic study of several concepts

An attempt was made to define more precisely the notion of spine instability in lumbar spondylolisthesis. By means of lateral radiographs in flexion and extnsion, the axes of movement and the degree of mobility at the L3--L4, L4--L5, and L5--S1 levels were determined in 24 cases of true (spondylolytic) spondylolisthesis. Axes of movement at the spondylolisthetic levels showed a somewhat larger spread in and around the discs than at the normal levels, but instability in the form of parallel displacement of vertebral endplates was not observed. Hypermobility at the spondylolisthetic level proved to be a regular finding. The still current concept of "instability" as presented by Knutsson in 1944 was critically reviewed and denied as valid. Imprecision in the literature regarding instability may partly be attributed to insufficient discrimination between vertebral displacement as anatomic relationship and as abnormal type of movement. By designing a two-dimensional model other movements than flexion-extension could be studied; a special form of instability at the spondylolisthetic level was shown to appear during forward and backward movement of the lumbar spine above it. This detailed analysis facilitates understanding of the effect of different surgical procedures on vertebral movement and instability in lumbar spondylolisthesis.