The segmental instability which follows as a result of disc degeneration and partial ligamentous failure sometimes produces a special type of lesion in the lower lumbar spine, commonly at the L4 level. Excessive mobility on flexion and extension contributes towards degenerative changes in the posterior joints which in turn allows forward and backward gliding of the involved vertebral bodies. Subluxation of the arthritic apophyseal joints permits forward displacement of one vertebral body on the adjacent vertebra below, and the displacement becomes fixed because of an increase in the angle between the pedicle and the inferior processes (Macnab, 1977) (Figure 17.1).