More than 85% of Americans experience back pain by age 50. Low-back pain is the second most common ailment in the United States headache tops the list and the second most common reason for absences from work and visits to a physician. Low-back pain is estimated to cost as much as $50 billion a year in lost productivity, medical and legal fees, and disability insurance and compensation.

Back pain can result from sudden traumatic injuries, but it is more often the long-term result of weak and inflexible muscles, poor posture, or poor body mechanics during activities like lifting and carrying. Any abnormal strain on the back can result in pain. Most cases of low-back pain clear up within a few weeks or months, but some people have recurrences or suffer from chronic pain.

Function and Structure of the Spine

The spinal column performs many important functions in the body.

• It provides structural support for the body, especially the thorax (upper-body cavity).

• It surrounds and protects the spinal cord.

• It supports much of the body’s weight.

• It serves as an attachment site for a large number of muscles, tendons, and ligaments.

• It allows movement of the neck and back in all directions.


The spinal column is made up of bones called vertebrae that provide structural support to the body and protect the spinal cord (Figure 5.3). The spine consists of 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the upper back, and 5 lumbar vertebrae in the lower back. The 9 vertebrae at the base of the spine are fused into two sections and form the sacrum and the coccyx (tailbone). The spine has four curves: the cervical, thoracic, lumbar, and sacral curves (see Figure 5.3). These curves help bring the body weight supported by the spine in line with the axis of the body.

Although the structure of vertebrae depends on their location on the spine, the different types of vertebrae share common characteristics. Each consists of a body, an arch, and several bony processes (Figure 5.4). The vertebral body is cylindrical, with flattened surfaces where intervertebral disks are attached. The vertebral body is designed to carry the stress of body weight and physical activity. The vertebral arch surrounds and protects the spinal cord. Irregularly shaped bony outgrowths serve as joints for adjacent vertebrae and attachment sites for muscles and ligaments. Nerve roots from the spinal cord pass through notches in the vertebral arch.

Intervertebral disks, which absorb and disperse the stresses placed on the spine, separate vertebrae from one The spinal column. The spine is made up of five separate regions and has four distinct curves. An intervertebral disk is located between adjoining vertebrae.

Disks are made up of a gel- and water-filled nucleus surrounded by a series of fibrous rings. The liquid nucleus can change shape when it is compressed, allowing the disk to absorb shock. The intervertebral disks also help maintain the spaces between vertebrae where the spinal nerve roots are located.

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