The design specification for the human vertebral column is the provision of structural stability affording full mobility as well as protection of the spinal cord and axial neural tissues.1 While achieving these seemingly disparate objectives, the spine also contributes to the functional requirements of gait and to the maintenance of static weight-bearing postures (see Chapter 6).1
At the component level, the basic building block of the spine is the vertebra. The vertebra serves as the weight-bearing unit of the vertebral column, and it is well designed for this purpose. Although a solid structure would provide the vertebral body with sufficient strength, especially for static loads, it would prove too heavy and would not have the necessary flexibility for dynamic load bearing.2 Instead, the vertebral body is constructed with a strong outer layer of cortical bone and a hollow cavity, the latter of which is reinforced by vertical and horizontal struts called trabeculae.
The term vertebral column describes the entire set of vertebrae excluding the ribs, sternum, and pelvis (Fig. 22-1). The normal vertebral column is made up of 29 vertebrae (7 cervical, 12 thoracics, 5 lumbar, and 5 sacral) and three or four coccygeal segments. The adage that “function follows form” is very much applicable when studying the vertebral column. Although all vertebrae have similar characteristics, each has specific details that reflect its unique function. The overall contour of the normal vertebral column in the coronal plane is straight. In contrast, the contour of the sagittal plane changes with development. At birth, a series of primary curves give a kyphotic posture to the whole spine. With the development of the erect posture, secondary curves develop in the cervical and lumbar spines, producing a lordosis in these regions. The curves in the spinal column provide it with increased flexibility and shock-absorbing capabilities.2
The vertebral column. (Reproduced, with permission, from Chapter 1. Back. In: Morton DA, Foreman K, Albertine KH. eds. The Big Picture: Gross Anatomy. New York, NY: McGraw-Hill; 2011.)
Functionally, the vertebral column consists of anterior and posterior columns. The anterior column is the hydraulic and weight-bearing portion that provides the vertebral column with its shock-absorbing capability and consists of the vertebral bodies and intervertebral disks (IVD).3 The posterior column consists of the structures that provide the gliding mechanism for movement, the articular processes, and the zygapophyseal (facet) joints.3 A motion segment in the vertebral column is defined as two adjacent vertebrae and consists of three joints. One joint is formed between the two vertebral bodies and the IVD of the anterior column. The other two joints are formed by the articulation of the superior articular processes of the inferior vertebra and the inferior articular processes of the superior vertebra (Fig. 22-2) of the posterior column.