Spondylolisthesis, i.e. slippage (olisthesis) of vertebral bodies with respect to each other may result from defects in the bony strut (see spondylolysis); this variety is often termed isthmic spondylolisthesis. Spondylolisthesis may also occur as a result of facet joint and ligamentous degenerative changes, which permit the force vectors acting on the spine to displace vertebrae out of their normal alignment. Because these forces are greatest in the lower part of the body, this form, also known as degenerative spondylolisthesis, is most commonly seen in the lower lumbar or lumbo-sacral region. Such malalignment can result in stretching of pain-sensitive tissues and compression of lumbar nerve roots.

Symptoms: Pain, and loss of nerve function which may result in weakness and numbness, are the most common symptoms. In severe spondylolisthesis, nerve roots that innervate bladder and rectum can be compressed resulting in sphincter control problems.

Diagnosis: Plain x-rays are very helpful because they can demonstrate malalignment of vertebrae in different positions. Flexion and extension films may demonstrate instability not otherwise apparent. Nerve root compression is best seen on MR scans.

Treatment: Surgical treatment usually requires fusion of the involved vertebrae in a corrected position. Occasionally, spondylolysis can be treated by a more local procedure to strengthen and fuse only the defect in the bony strut.