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Formed by 33 vertebrae and divided into four regions, the spinal column and its associated muscles provide postural control to the torso and skull, while also protecting the spinal cord. The conflicting needs for range of motion (ROM) versus protection of the spinal cord are met in varying degrees throughout the spine (Fig. 13-1).
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The four regions of the spinal column provide different levels of mobility and protection to the spinal cord. The cervical spine provides the greatest ROM; however, this results in the most vulnerability to the spinal cord relative to the other, lesser mobile, regions of the spine. The thoracic spine provides the greatest protection of the spinal cord, but it does so at the expense of ROM. The lumbar spine provides a more equal balance between protection of the spinal cord and available ROM. The sacrum and coccyx are composed of fused bones. The sacrum affixes the spinal column to the pelvis and serves as a site for muscle attachment. At this level, the spinal cord has exited the column.
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The body’s weight is transmitted primarily along the spinal column via the vertebral bodies, whose size is related to the amount of force they transmit. Carrying only the weight of the head, the vertebral bodies of the cervical vertebrae are much smaller than the lumbar vertebrae, which are required to transmit and absorb the weight of the entire torso. A segment comprises two adjacent vertebral bodies, the intervening disc, and the soft tissue that holds the vertebrae together. This represents the smallest functional unit of the spine. The vertebrae of the cervical spine are included in this section for a basis of comparison. Refer to Chapter 14 for further description of the cervical spine.
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Figure 13-2 compares the relative sizes of the cervical (n = 7), thoracic (n = 12), and lumbar (n = 5) vertebrae and identifies the bony landmarks. Each vertebra, with the exception of the first cervical vertebra, has a distinct body that is situated anteriorly and comprises the primary weight-bearing surface. Projecting immediately posteriorly from the body are the sturdy pedicles, forming the anterior portion of the neural arch. The posterior portion of the arch is composed of the lamina, articular processes, transverse processes, and spinous processes. The laterally projecting transverse processes, arising from the laminae, provide an attachment site for the spine’s intrinsic ligaments and muscles and increase the muscles’ mechanical leverage. The prominent posterior projections, the spinous processes, act as attachment sites for muscles and ligaments. Their angulation relative to the vertebrae below limits ...