A compression fracture is usually understood to mean a compression fracture of the spine, or vertebral compression fracture. A compression fracture occurs when a number of vertebrae in the spine are broken. It may be due to trauma or due to a weakened vertebra in a patient with osteoporosis or osteogenesis imperfecta. In healthy patients it is most often seen in individuals suffering extreme vertical shocks, as from plane ejection seats, or falling a considerable distance. Seen in lateral views in plain x-ray films, compression fractures of the spine characteristically appear as wedge deformities, with greater loss of height anteriorly than posteriorly and intact pedicles in the anteroposterior view.

The spine consists of 33 bones, known as vertebrae, separated into four regions. The uppermost seven vertebrae are known as the cervical vertebrae and extend down to the upper chest. The next twelve bones are known as the thoracic vertebrae and extend to the lower back. The next five are the sturdiest and largest, and are known as the lumbar vertebrae, making up the lower back between the hips and chest. The final nine bones are fused into two segments, the coccyx and the sacrum.

A compression fracture can occur throughout the spine, but most commonly affects two or more vertebrae within the lower thoracic and upper lumbar region, sometimes referred to as the thoracolumbar section of the back. A compression fracture is nearly always a serious injury, because the spinal cord is near each of the vertebrae and is at risk whenever they shift. A compression fracture is most likely to be caused by serious trauma to the spine, particularly in the case of falling from an extreme height. Because of the force necessary to cause compression fractures, they often occur in tandem with other injuries, including damage to the spinal cord and severe ligament damage. Nearly one-in-a-thousand people experience a compression fracture at some point in their life, though the level of damage varies greatly.

Adopted from wisegeek.com and wikipedia.com