Like cervical stenosis, lumbar spinal stenosis is frequently precipitated by disc degeneration with subsequent marginal osteophytosis of the vertebral body ends, hypertrophic degeneration of the facet joints, and bulging of the ligamenta flava. Lumbar stenosis may be lateral, central, or combined. The lower lumbar vertebrae normally have shorter pedicles that cause the superior articular processes to intrude into the spinal canal to cut off narrow lateral recesses (Fig. 6-61A–E). The lateral nerve roots of the next spinal nerve to exit as they descend recesses are bordered by the pedicles laterally, the vertebral within their dural sleeve. Osteophytes that develop on the body anteriorly, and, most important, the superior articular anteromedial margin of the superior articular processes of the processes posteriorly. The lateral recesses are occupied by the next lower vertebra are most likely to encroach on the lateral recess to produce lateral stenosis. Because the inferior articular processes of the next higher vertebra are situated posteromedial to the superior articular processes, osteophytes developing on their anterior margin are more likely to produce central steno- sis. In central stenosis, any or all of the rootlets of the cauda equina can be encroached on. Vertebral body margin osteophytes and buckling of the ligamentum flavum can contribute to lumbar spinal stenosis.
FIGURE 6-61. MDCT of the lumbar spine in a patient with degenerative spondylosis and spinal canal steno- sis. At the L4-5 level, there are hypertrophic degenerative changes of the facet joints and ligamentum flavum (long white arrow), which produce lateral stenosis, and hypertrophic changes of the articular process, which cause central stenosis (short white arrow). A: Bone windows. B: Soft-tissue windows that demonstrate to a better advantage the hypertrophied ligamentum flavum (short arrow ). C: Soft-tissue windows and D: bone windows. There is congenital canal stenosis as well. Compression to the superior end plate of L1 is appreciated. E: Volume rendering reconstruction looking at the central canal demonstrates the significant narrowing to the central canal.
Hypertrophic degenerative changes involving the facet joints can also encroach on the posterior aspect of the neural foramen and produce foraminal stenosis with compression of the nerve roots exiting that foramen. Therefore, hypertrophic degenerative changes involving a single superior articular process can involve the roots of two closely adjacent nerves, with the possibility of producing both foraminal and lateral spinal stenosis. With disc degeneration and loss of disc height, the neural foramen can be further compromised by the upward and forward displacement of the superior articular process into the upper part of the neural foramen, where the nerve roots are situated. In addition, because of the obliquity of the facet joint, the accompanying downward displacement of the inferior articular process of the next higher vertebra can produce retrolisthesis (i.e., backward displacement) of its vertebral body into the upper portion of the neural foramen.
By standard myelography, the protruding disc anteriorly and the bulging ligamenta flava posteriorly can produce an hourglass appearance of the thecal sac (Fig. 6-62). By CT, all osteophytes are clearly visualized, and measurements of the AP dimension of lateral recesses that are less than 3 mm are strongly suggestive of lateral stenosis (69). The hypertrophic changes producing central and foraminal stenosis are also well visualized. Sagittal reformations are especially helpful in evaluating foraminal stenosis.
FIGURE 6-62. Lumbar spinal stenosis caused by both protruding process of L5 cause shearing stresses to be concentrated on the pars discs and bulging ligamentum flavum. Lateral myelogram showing the interarticularis of L5 (curved arrows). This produces the stress fracture hourglass appearance of the thecal sac. of spondylolysis.
Facet anatomy is well seen by MRI, with subchondral bone appearing as a signal void. On T1-weighted images, articular cartilage is visualized as a moderate–signal-intensity interval between the subchondral bone of the two articular processes. This becomes more signal intense on T2-weighted images. Facet joint degeneration appears as an irregularity or reduction in the thickness of the articular cartilage. Osteophytes are usually displayed as signal voids encroaching into the foramen, lateral recess, or spinal canal. Occasionally, osteophytes show high– signal-intensity interior, indicating the presence of marrow.