At the level of the intervertebral foramen, the spinal nerve roots join and become a spinal nerve. There are seven lumbar
spinal nerves. The individual lumbar spinal nerves exit through the intervertebral foramen caudal to the vertebra of the corresponding
number. For example, the L6 spinal nerve exits through the intervertebral foramen formed by the L6 and L7 vertebrae, while
the L7 spinal nerve exits through the intervertebral foramen formed by the L7 vertebra and the sacrum. Spinal nerve roots
that originate more cranially will lie more laterally within the vertebral canal. Therefore, L7 spinal nerve roots are located
lateral to the S1 spinal nerve roots, which are lateral to S2 spinal nerve roots, and so forth. Once outside the vertebral
canal, the L6, L7, and S1 spinal nerves (with occasional contributions from the L5 and S2 spinal nerves) converge to become
the sciatic nerve.4,7 The S1-S3 spinal nerves come together to form the pelvic and pudendal nerves, while the caudal nerves innervate the tail.4
Bony, cartilaginous, ligamentous, and vascular structures
1. A T2-weighted magnetic resonance image of the caudal aspect of the L7 vertebra in the axial plane. Note the lateral recesses
(arrows). The asterisk is overlying the body of the L7 vertebra.
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In addition to the neural structures, a strong understanding of the bony, cartilaginous, ligamentous, and vascular structures
that make up the vertebral column from the L7 vertebra through the sacrum is important. The vertebral canal in the L7 vertebra
and sacrum tends to be dorsoventrally flattened compared with elsewhere in the vertebral column. Lateral depressions in the
vertebral canal within the L7 body form lateral recesses for the L7 spinal nerve to exit the intervertebral foramen just cranial
to the L7-S1 intervertebral disk (Figure 1). At the level of the intervertebral disk, the vertebral canal is composed of aspects of the L7 vertebra, intervertebral
disk, interarcuate ligament, and sacrum. The dorsal portion of the vertebral canal is composed of the dorsal lamina of the
L7 vertebra and the sacrum as well as the interarcuate ligament that spans the space between the vertebrae. The ventral portion
of the canal is composed of the body of the L7 vertebra, the dorsal anulus fibrosus of the intervertebral disk, and the body
of the sacrum. Overlying these structures is the dorsal longitudinal ligament. The articulations between the articular processes
and facets of the L7 vertebra and the sacrum form synovial joints that dorsolaterally overlie the vertebral canal.
The vascular structures also contribute to the pathophysiology of degenerative lumbosacral stenosis. The internal vertebral
venous plexus (often referred to as the vertebral venous sinus) lies on the ventral aspect of the vertebral canal. This paired venous structure winds its way down the entire vertebral
canal, diverging laterally at the intervertebral disk and converging within the body of the vertebra, often interconnecting
at the convergence. In addition to the spinal nerves, spinal arteries and veins traverse the intervertebral foramen. The intervertebral
foramen is more akin to a canal.8 It is formed by the caudal vertebral notch of the L7 vertebra and the cranial notch of the sacrum (cranially and caudally),
the intervening intervertebral disk (ventrally), and the articular processes and synovial joint capsules (dorsally).
PATHOLOGIC FINDINGS
Degenerative lumbosacral stenosis involves varying degrees of pathologic changes of several anatomical structures. Abnormalities
include
- Chronic Hansen's Type II degenerative intervertebral disk disease of the L7-S1 intervertebral disk
- Hypertrophy and ventral folding of the interarcuate ligament
- Osteoarthritis and subsequent joint capsule proliferation of the articular facets of L7-S1 articulation
- Subluxation of the sacrum in relation to the L7 vertebra
Occasionally, osteochondrosis of the cranial sacral or caudal L7 end plate may be seen.9,10
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