Adult Slip Progression
In contrast to the well-documented slip progression (increase in the extent of anterior vertebral slippage) that may occur in children and adolescence, late progression of isthmic spondylolisthesis in adults is rarely discussed or described in spine literature. In the last decade, adult onset slip progression was recognized and described in the medical literature as a distinct clinical entity causing severe incapacitating back and leg pain (1). Moreover, it was found that slip progression is not at all rare. It is always accompanied by disc degeneration at the slip level. As the disc loses it structural and functional integrity, the lumbosacral junction (L5-S1) becomes unstable and the slip progresses.

The following figures exemplify isthmic slip progression in adults (Fig. 5a, 5b, 5c). These serial lumbar spine radiographs (x-rays) demonstrate the progressive nature of the slip. The increased spondylolisthesis always coincides with disc degeneration at the slip level.

Figure 5a. Serial radiographs (x-rays) demonstrating progression
of vertebral slippage in the lumbar spine at age 34, 40, and 47.

Figure 5b. Lateral x-rays indicating isthmic spondylolisthesis.

Figure 5c. Additional lateral x-rays indicating isthmic spondylolisthesis.

Slip progression occurs in about 20% of adults with isthmic spondylolisthesis. Slip progression starts usually after the third decade of life and coincides with marked disc degeneration at the olisthetic (downward slipping) level. Slip progression is associated with clinical signs of mechanical instability and spinal stenosis manifested by incapacitating low back pain and significant sciatica. The concurrent occurrence of disc degeneration at the slip level and adult slip progression explains how an asymptomatic developmental lesion, present for at least two or three decades, may become symptomatic.

Reference
1. Floman Y. Spine 25:342, 2000.