Postoperative instability after decompression for lumbar spinal stenosis.

Slipping after decompression for myelographically verified spinal stenosis (AP diameter less than or equal to 11 mm) was studied in 45 patients (32 men and 13 women). Mean age at the time of operation was 64 years. Mean time of postoperative observation was 46 months. The results in 29 patients were excellent or good and in 16 unchanged or worse. Degenerative spondylolisthesis was found in 20 patients and acquired spinal stenosis in 25. Postoperative slipping was seen in 18 patients. Seven belonged to the good group. A significant increase of patients with slipping was seen in the poor group (P less than 0.01). An enhanced risk of further slipping was seen in degenerative spondylolisthesis (P less than 0.01), but it did not influence the result of the operation. The radicality of the operation was about equal in both the good and poor groups and in the groups with or without postoperative slipping. A general decrease of the disc heights after operation was seen in all groups but was not significant in the poor group.