There are seven cervical, twelve thoracic, five lumbar, and five sacral vertebrae. The coccyx consists of up to five fused segments. Vertebral shape and size vary in different regions of the spine. Figures 1-1A, 1-1B, and 1-1C demonstrate typical cervical, thoracic, and lumbar vertebrae, respectively, as seen from the front, in a lateral view, and from above. It can be clearly seen that the lower vertebrae, which carry a heavier load, have larger and stronger vertebral bodies.

The pedicles connect the vertebral bodies to the posterior elements—laminae and spinous processes. Their contours can be clearly seen in plain films in the AP views. The superior and inferior articular facets are also clearly seen (Figures 1-2A, 1-2B, and 1-2C).

The orientation of the facet joints, which is different in the cervical, thoracic, and lumbar regions, has a great impact on spinal stability and mobility In the cervical region the superior articular facets face cranially and posteriorly, thus providing some stability in forward motion while allowing significant mobility. The thoracic facets are oriented in the coronal plane, thus preventing forward motion, whereas the lumbar facets are oriented mostly in the sagittal plane, preventing axial rotation and lateral movement of one vertebra in relation to neighboring vertebrae while allowing flexion and extension. The transverse processes can be identified in plain films. A progressive increase in the space between the articular facets is noted in the caudal end of the spinal column. This allows for the development of lumbar lordosis. The transverse processes of the upper six cervical vertebrae contain an additional foramen, the foramen transversarium, in order to accommodate the vertebral arteries. The transverse process of the seventh cervical vertebra does not have a foramen transversarium (Figure 1-3).

The spinal canal occupies the space between the posterior wall of the vertebral bodies and the discs, and the anterior aspect of the laminae and

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