Stenosis of lumbar intervertebral foramen: anatomic study on predisposing factors.

Study Design: In Study 1 the authors measured the vertical and sagittal dimensions of the intervertebral foramen in dried lumbar vertebrae. In Study 2 the dimensions of the intervertebral foramen were measured in fresh cadaveric spines in which the soft tissues forming the foraminal walls were preserved.
Objectives: To investigate whether disc narrowing, or other factors related to the morphometry of lumbar vertebrae, may predispose to foraminal stenosis.
Summary of Background Data: Predisposing factors to foraminal stenosis have been little investigated. Previous studies suggested that narrowing of the disc space might cause a reduction in the foraminal dimensions; however, it is not clear whether such a reduction is to such an extent as to compress the nerve root within the foramen.
Methods: In Study 1 the vertical and sagittal dimensions of 160 intervertebral foramens were measured on dried white spines. Measurements were performed in the presence of a normal height of the disc space, obtained using a silicone rubber disc, and repeated in the same specimen after disc removal. Foraminal dimensions were then related to the sagittal diameter of the spinal canal and the pedicle length. In Study 2, the authors evaluated the dimensions of 50 intervertebral foramens of fresh cadaveric spines, in which the soft tissues forming the foraminal walls and the nerve root were preserved. In specimens showing normal or slightly decreased disc height, the foraminal dimensions were measured before and after disc excision.
Results: In Study 1 the reduction in the foraminal height after disc removal was, on average, 6.5 mm (P < 0.0001). In both Studies 1 and 2 the sagittal dimensions of the foramen were not significantly reduced after disc removal. No significant difference was found in Study 1 in the sagittal dimensions of the foramen between specimens showing normal or degenerated facet joints. In Study 2 a compression of the nerve root within the foramen was found in one specimen, showing a concomitant central stenosis at multiple levels. A significant correlation was found between the sagittal diameters of the foramen and the sagittal diameter of the spinal canal and the pedicle length.
Conclusions: Narrowing of the disc space significantly reduces the vertical diameter of the foramen but has no significant effects on its sagittal dimensions. In contrast, the sagittal dimensions of the foramen are strictly related to the sagittal diameter of the spinal canal and the pedicle length. These results suggest that in patients with developmental or combined stenosis of the central spinal canal, a concomitant foraminal stenosis is likely to be present, or at least should be suspected.