Positional changes in lumbar disc herniation during standing or lumbar extension: a cross-sectional weight-bearing MRI study.

To investigate biomechanical changes in lumbar disc herniations. Patients with lumbar disc herniation verified on a 1.5–3-T magnetic resonance imaging (MRI) scanner were imaged in a weight-bearing 0.25-T MRI scanner in (1) standing position, (2) conventional supine position with relative lumbar flexion, and (3) supine position with a forced lumbar extension by adding a lumbar pillow. The L2-S1 lordosis angle, the disc cross-sectional area, the disc cross-sectional diameter, and the spinal canal cross-sectional diameter were measured for each position. Disc degeneration and nerve root compression were graded, and the pain intensity was reported during each scan position. Forty-three herniated discs in 37 patients (36.7 ± 11.9 years) were analyzed in each position. The L2-S1 lumbar angle increased in the standing position (mean difference [MD]: 5.61°, 95% confidence interval [95% CI]: 3.44 to 7.78) and with the lumbar pillow in the supine position (MD: 14.63°, 95% CI: 11.71 to 17.57), both compared with the conventional supine position. The herniated disc cross-sectional area and diameter increased during standing compared with during conventional supine position. No changes were found in the spinal canal cross-sectional diameter between positions. Higher nerve root compression grades for paracentral herniations were found during standing compared with during conventional supine position. This was neither found with a lumbar pillow nor for central herniations in any position compared with conventional supine. Disc herniations displayed dynamic behavior with morphological changes in the standing position, leading to higher nerve root compression grades for paracentral herniated discs. • Lumbar herniated discs increased in size in the axial plane during standing. • Increased nerve root compression grades for paracentral herniated discs were found during standing. • Weight-bearing MRI may increase the diagnostic sensitivity of nerve root compression in lumbar disc herniations.