The intervertebral disc is anatomically composed of a central nucleus pulposus with a peripheral and circumferential annulus fibrosus (Figure 7.1).2 The nucleus is bordered superiorly and inferiorly by the car
Figure 7.1. Sagittal diagram showing the anatomy of the lumbar intervertebral disc. The soft inner nucleus pulposus is encircled by the fibrous bands of the annulus which are thinner posteriorly.
tilaginous endplates on the articular surface of the adjacent vertebrae. The annulus itself is composed of two layers, an inner layer that attaches to the cartilaginous endplates, and an outer ligamentous layer that attaches directly to bone of the vertebral bodies. The annulus is loosely attached to the anterior longitudinal ligament but densely adherent to the posterior longitudinal ligament. Annular fibers are thicker anteriorly. The nucleus is a notochordal remnant that is relatively avas-cular in the adult and is not significantly innervated. The role of the nucleus in back pain is believed to be primarily a consequence of mechanical mass effect or chemical effects on local innervated structures. The annulus fibrosus, however, is innervated, most densely along the posterior aspect, and substance P and unmyelinated C fibers have been demonstrated in the annulus, supplied by way of the sinovertebral nerve (Figure 7.2).14-16 Sympathetic fibers are also evident adjacent to the outer portions of the annulus.
The function of the intervertebral disc is a combination of stress absorption (primarily nucleus), and motion restriction (annulus).2 The an-
Figure 7.2. Axial cross section showing the innervation of the intervertebral disc. There is no significant innervation of the nucleus, while the annulus is innervated with un-myelinated fibers, primarily by way of the sinovertebral nerve. Pain fibers are present throughout the disc but most densely in the posterior annulus.
nulus serves to contain the nuclear material and to restrict longitudinal and rotational motion between spinal segments. Fibers in the annulus are arranged in variable directions in each fibrous layer (approximately 20 anteriorly, and approximately 12-15 posteriorly), providing support in multiple directions.
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