Magnetic resonance imaging evaluation of thecal sac diameters following laminectomy in the lumbar spine.

Sagittal magnetic resonance images (MRIs) from 20 patients who had postoperative persistent or recurrent low back pain were reviewed to study the thecal sac diameter at laminectomy sites. The thecal sac anteroposterior diameter was measured in three sites: the maximum AP diameter at the laminectomy site, and the proximal and distal levels to the laminectomy site. A comparison between the thecal sac AP diameter at laminectomy sites and the average diameter of both the proximal and distal levels was then made. Based on the measurements, the diameters of the thecal sac were divided into three groups. Patients in group A (5 cases) had a constriction (the thecal sac AP diameter at the laminectomy site was less than the average diameter of both the proximal and distal levels) of the thecal sac at the laminectomy site. Patients in group B (12 cases) had an expansion (the thecal sac AP diameter at the laminectomy site was more than the average diameter of both the proximal and distal levels) of the thecal sac. Patients in group C (3 cases) demonstrated no change. This study suggests that there was no correlation between the thecal sac AP diameter at laminectomy site and postoperative low back pain.