Segmental instability. Rationale for treatment.

Segmental instability, secondary to spinal degeneration, is a controversial topic. Based on current clinical, radiographic, and biomechanical considerations, this condition is classified as axial rotational, translational, retrolisthetic, and postsurgical instability syndromes. Each of these conditions would be expected, if untreated, to progress to a fixed deformity in which the clinical symptoms of spinal stenosis would predominate. The classification of these four types of instability suggests a need for specifically tailored fusion techniques in those patients who fail to respond to conservative treatment. Antitorsion facet fusion is suggested for axial rotatory instabilities; anterior (or posterior) interbody fusion for translational instabilities; and fusion in flexion (Knodt rods or facet fusion) for retrolisthetic instabilities. Post-surgical instability syndromes require carefully selected approaches based on the overall pathology. Application of specific fusion techniques, in carefully selected patients, may improve the currently unacceptable low rate of success from such operations.