The effect of body position and axial load on spinal canal morphology: an MRI study of central spinal stenosis.

Study Design. A method comparison study. Objective. To investigate the effect of body position and axial load of the lumbar spine on disc height, lumbar lordosis, and dural sac cross-sectional area (DCSA). Summary of Background Data. The effects of flexion and extension on spinal canal diameters and DCSA are well documented. However, the effects of axial loading, achieved by upright standing or by a compression device, are still unclear. Methods. Patients with lumbar spinal stenosis were examined in 2 separate studies, including 16 and 20 patients, respectively. In section 1, magnetic resonance imaging (MRI) scans were performed during upright standing and supine positions with and without axial load. In section 2, MRI scans were performed exclusively in supine positions, one with flexion of the lumbar spine (psoas-relaxed position), an extended position (legs straight), and an extended position with applied axial loading. Disc height, lumbar lordosis, and DCSA were measured and the different positions were compared. Results. In section 1, the only significant difference between positions was a reduced lumbar lordosis during standing when compared with lying (P = 0.04), most probably a consequence of precautions taken to secure immobility during the vertical scans. This seemingly makes our standing posture less valuable as a standard of reference. In section 2, DCSA was reduced at all 5 lumbar levels after extension, and further reduced at 2 levels after adding compression (P < 0.05). Significant reductions of disc height were found at 3 motion segments and of DCSA at 11 segments after compression, but these changes were never seen in the same motion segment. Conclusion. Horizontal MRI with the patient supine and the legs straightened was comparable to vertical MRI whether axial compression was added or not. Extension was the dominant cause rather than compression in reducing DCSA. Axial load was not considered to have a clinically relevant effect on spinal canal diameters.