Schematic representation of anteroposterior, lateral, and craniocaudal (top to bottom) views of typical (A) cervical, (B) thoracic, and (C) lumbar vertebrae.

the ligamenta flava that are adjacent to them. The relationship between the canal size and the size of its contents, the neural elements, is of paramount importance. In tight spots such as the thoracic spine region, the spinal cord fills most of the available space. Here, even a moderate-sized space-occupying lesion, such as a herniated disc or tumor, may lead to progressive, severe neurological compromise due to cord compression. In the lumbar region, however, where the canal is wider, lesions of the same size may remain asymptomatic for a long time because there is ample space left for the nerve roots to move away from the offending structure.

Spinal Canal and Neural Elements

The spinal canal contains the spinal cord and the nerve roots. The spinal cord, which is the continuation of the brain stem, extends from the foramen magnum to the L1-L2 level. The lower tip of the spinal cord—the conus medullaris—is a cone-shaped structure pointing downward that contains the centers for micturition and defecation. Because it is shorter than the spinal column, its lower segments do not correspond with the vertebrae at the same level. Below the L1-L2 level the spinal canal contains the lumbar and sacral nerve roots, which exit through their respective foraminae. As they course down toward their exit points they form the cauda equina.

There are eight cervical nerves. The first through the seventh cervical nerves exit above their respective vertebrae. The eighth cervi cal nerve exits the spine between C7 and the first thoracic vertebra (Figure 1-4). As a result, all the nerves below C8 (thoracic, lumbar, sacral) exit the spine below their respective vertebrae (e.g., T7 nerve root exits under T7 vertebral body).

The nerves exit the spine through the intervertebral foraminae. The anterior "wall" of the foramen is formed by the posterolateral region of the vertebral bodies and discs. The "ceiling" and "floor" are formed by the pedicles, and the posterior "wall" is formed by the facet joints. The dorsal root ganglia are located within the intervertebral foraminae (Figure 1-5).

The Cervical Spine

The first two cervical vertebrae, C1 and C2, are known as the atlas and the axis, respectively. C1 and C2 are anatomically very different from the rest of the cervical vertebrae. The first cervical vertebra, the atlas, does not have a vertebral body per se; it has been incorporated into the dens. It consists of a ring. The anterior arch of this ring is rather short and stays in close proximity to the dens. The posterior arch of the atlas is much longer in order to accommodate the spinal cord (Figures 1-6A, 1-6B, and 1-6C).

The superior articular facets of C1 articulate with the occipital condyles, and the inferior articular facets articulate with C2. There is no intervertebral disc between the C1 and C2 vertebrae. The second cervical vertebra, the axis, carries the odontoid process upon which the atlas rotates. The odontoid process "articulates" with the posterior aspect of the anterior ring of the atlas and remains close to it in all positions, including flexion. It is kept in this position by the transverse ligament. The atlanto-axial complex can be well visualized in plain films in lateral and open mouth views (Figures 1-7A and 1-7B). Finer details of this region can be obtained with CT and/or MRI scans.

The C3 through C7 vertebrae have a quadrangular shape with sagittal diameter shorter than the coronal one. They bear a ridge on their superior posterolateral aspect—the uncinate process. These ridges add lateral stability to the mobile cervical spine and protect the exiting nerve roots from lateral disc herniations (Figure 1-8).

The "articulations" between the uncinate processes and the vertebral bodies above them, the joints of Luschka, are not true synovial joints.

The cervical nerve roots exit the spine between the uncinate processes and the facet joints. Uncinate process degeneration is a common cause of nerve root compression and may result in cervical radiculopathy. The cervical intervertebral foraminae are not well demonstrated on plain lateral films. In order to better visualize them, an oblique view should be obtained. Normally the foraminae look like inverted tear drops. The nerve roots are located in the upper one-third of the foramen and are surrounded by fatty tissue. The foraminae may be well demonstrated on oblique plain films and axial CT and MR images (Figures 1-9A and 1-9B).

The diameter of the cervical spinal canal can be roughly estimated on lateral views of the cervical spine. The anterior border of the canal can be easily identified: it is the posterior border of the vertebral bodies. The

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