Intervertebral discs join all the adjacent vertebral bodies between C2 and the sacrum. The outer portion of the articular surface of the bodies is a rim of epiphyseal bone, while the central portion of the articular surface is lined with hyaline cartilage. Each intervertebral disc consists of a fibrocartilaginous rim, the annulus fibrosis, and a centrally placed mass of gelatinous material, the nucleus pulposus (Fig. 8). The external fibers of the disc crisscross as they pass from the epiphyseal rim of one body to the next. The criss-crossing pattern of the fibers permits some anterior and posterior displacement and some rotation between one vertebra and the next. All but the most peripheral part of the perimeter of the discs is avascular, and exchange of nutrients and metabolic wastes occurs through the articular hyaline cartilage into the cancellous bone of the vertebral bodies. There is a great deal of interest in determining how nutrition of the intervertebral discs is accomplished, and techniques in magnetic imaging have
been developed to study this process. Using a combination of multiple administration of contrast medium, delayed timing of scanning, and a highly sensitive T1-weighted sequence, it has been possible to visualize solute transport into and within the intervertebral disc (5).
Mercer and Bogduk have described an important difference between the cervical and lumbar annulus fibrosis (6). The cervical annulus fibrosis does not consist of concentric laminae of collagen fibers as it does in the lumbar region. Instead, the cervical annulus forms a thick, cres-centic mass of fibers anteriorly that taper laterally toward the uncinate process. The cervical annulus is essentially deficient posterolaterally and is represented posteriorly by only a thin layer of vertically oriented fibers. These findings have implications for understanding cervical disc function, imaging, and pathology.
In the static state, the anterior and posterior longitudinal ligaments reinforce the union of adjacent vertebral bodies and support the intervertebral discs. In the dynamic state, the anterior longitudinal ligament helps prevent hyperextension and the posterior longitudinal ligament helps prevent hyperflexion. The anterior longitudinal ligament is a broad, flat band that extends from the anterior tubercle of the atlas to the pelvic surface of the sacrum. The anterior ligament has a superficial layer of fibers that are long and a deep layer of fibers that extend over only one or two vertebrae. The posterior longitudinal ligament is located within the vertebral canal on the posterior aspect of the vertebral bodies and intervertebral discs. Superiorly, as the tectorial membrane, the posterior longitudinal ligament covers the transverse ligament of the atlas and attaches to the occipital bone. In the thoracic and lumbar regions, the posterior ligament is broader over the inter-
vertebral discs and narrower over the vertebral bodies, giving a serrated appearance to its lateral margins. Inferi-orly, the posterior longitudinal ligament is attached within the sacral canal. The deficiency of the posterior longitudinal ligament over the posterolateral aspect of the lumbar intervertebral discs contributes to herniation of the nucleus pulposus in the lumbar region (Fig. 9).
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