The narrowing of the lumbar spinal canal during loaded MRI: the effects of the disc and ligamentum flavum.

Load and activity changes of the spine typically cause symptoms of nerve root compression in subjects with spinal stenosis. Protrusion of the intervertebral disc has been regarded as the main cause of the compression. The objective was to determine the changes in the size of the lumbar spinal canal and especially those caused by the ligamentum flavum and the disc during loaded MRI. For this purpose an interventional clinical study on consecutive patients was made. The lumbar spines in 24 supine patients were examined with MRI: first without any external load and then with an axial load corresponding to half the body weight. The effect of the load was determined through the cross-sectional areas of the spinal canal and the ligamentum flavum, the thickness of ligamentum flavum, the posterior bulge of the disc and the intervertebral angle. External load decreased the size of the spinal canal. Bulging of the ligamentum flavum contributed to between 50 and 85% of the spinal canal narrowing. It was concluded that the ligamentum flavum, not the disc had a dominating role for the load induced narrowing of the lumbar spinal canal, a finding that can improve the understanding of the patho-physiology in spinal stenosis.