Instability of the lumbar spine.

It is convenient to divide the spectrum of clinical manifestation of degenerative change seen in the lumbar spine into three stages: ( 1 ) temporary dysfunction; (2) unstable phase; and ( 3 ) stabilization. The duration of each stage varies greatly and there is no clear-cut division between the clinical signs and symptoms of one stage and the next. However, it is important to maintain an overall perspective and, within this frame, attempt to delineate the condition or groups of conditions often referred to as instability.