Anatomy

What structures are most commonly injured?

The lumbar spine is the section of the spine commonly called the low back. It consists of a stack of five small bones (vertebrae) one on top of the other. The section of the spine below this region is called the sacrum. The sacrum consist of several vertebrae bonded together. The section above the lumbar spine is the thoracic spine. Each vertebra in that section has a rib attached.

All lumbar vertebrae are very similar. There is a drum like vertebral body in the front that takes most of the weight. Just behind that is an arch of bone protecting the nerves. Sticking out at the back is the spinous process. If you run your fingers down the middle of your back, the hard bumps you can feel are the spinous processes of the individual vertebrae. The transverse processes that jut out from either side of the arch cannot be felt. These “processes” are where the muscles that move the spine are attached.

The facet joints link one vertebra to the next and allow for some movement. When you bend forward or bend side-to-side, much of the movement between individual vertebrae occurs at the facet joints. Because the facet joints are set at an angle not much rotation can occur in this section of the spine. The rest of the movement of the spine takes place between the vertebral bodies. The area between each vertebra is occupied by the intervertebral disc. The intervertebral disc is made of two parts: the annulus, a ring of tough fibrous tissue, and a nucleus, the jelly-like material in the middle. The disc allows some pivoting between the next-door vertebral bodies. It also acts as a shock absorber when the spine is loaded.

Only a limited amount of movement occurs between each vertebrae. But, because there are five vertebrae, this section of the spine is quite flexible and compressible overall.

Ligaments, tough inelastic bands of fibrous tissue that pass from one bone to the next, hold the vertebrae together. They connect the vertebral bodies (longitudinal ligaments), the arches (ligamentum flavum) and the facet joints (capsule). They can be torn if the joints are dislocated or the bony attachments broken. Since ligaments do not show on x-ray, injury to ligaments cannot be seen directly. An abnormal gap between the parts of the joint shown on x-ray may be a sign of that the ligaments are torn.

Muscles also connect vertebra together. Muscles move the spine. They also contract to stop movement, or stabilize, the spine. For example, the muscles stabilize our spine when we walk. Much of the time your backbone holds still, as a support for the upper body. We don’t think about this; it is automatic.

Sometimes when we are suddenly shifted off balance the force of the automatic correction is too great for the bone and a small piece of bone is pulled off the spine. This is called an avulsion fracture. It is also important to remember that a force great enough to break a bone is more than enough to tear muscle. With every bony injury to the spine there is usually an accompanying injury to muscle and possibly to ligaments and nerves as well. This is called a “soft tissue” injury to distinguish it from the bony (hard tissue) injury.

The neural connection between the brain and the legs is called the spinal cord. It is best thought of as a part of the brain extending down inside the column of the spine. At each level the spinal cord gives off a set of nerve roots that exit the spine passing between the vertebrae.