Low Back Disc Disease and Herniated Discs

Anatomy of the Low Back
The lumbar spine is made up of the last five vertebrae of the spine. The vertebrae are the bones of the spine. Their function is to provide support and protection to the spinal cord. The facet joints allow the vertebrae to be linked. They provide mobile connections between each vertebra. An intervertebral disc sits between each individual vertebra. The annulus is the outer ring and is the strongest part of the disc. It is responsible for connecting the vertebrae. The nucleus pulposus is the soft, inner portion. This material is about the consistency of crabmeat and is responsible for the shock absorption properties of the spine.

The nerve roots of the spine carry signals between the lower extremities and the brain that allow us to move our legs and perceive sensations such as touch, temperature, and pain. To better understand how the parts of the spine affect each other, we sometimes focus on a spinal segment. A spinal segment is composed of two vertebra, the intervertebral disc between, and the two nerve roots that exit from that spinal level, one from each side.

The intervertebral discs have cartilaginous endplates at the top and bottom and are surrounded by the annulus. Through degeneration or injury, the fibrous tissue (annulus fibrosus) constraining the soft disc material (nucleus pulposus) may tear. This may result in bulging (protrusion) of the disc or even extrusion of disc material into the spinal canal or neural foramen. This condition has been called herniated disc, ruptured disc, herniated nucleus pulposus, or prolapsed disc.

Herniated Discs
One of the more common problems of the lumbar spine is a herniated disc. In this condition, a tear in an annulus fibrosus allows the nucleus pulposus to squeeze into the spinal canal. If a nerve root is compressed by the disc material, there can be pain, numbness, and weakness in the areas supplied by the nerve (often down the back of a leg). It is not unusual for the back itself to be painless, or nearly so. Accordingly, a herniated lumbar disc characteristically produces buttock and leg pain but not back pain per se. This pain is termed radiculopathy or sciatica.

If sensory function of the impinged nerve root is impaired, numbness will result. The exact area of numbness is determined by the particular root, and may be in the big toe, the heel, the outer ankle, the outer leg, or a combination of these. Impairment of motor function of the root will cause weakness which again depends on the particular root.

In disc herniations, the L5-S1 disc is involved 45% to 50% of the time, L4-5 40% to 45%, and L3-4 about 5%. Disc herniation at the other lumbar levels is rare.

The root compressed is the one exiting the level below the disc in the vast majority of cases. However, if the herniation is lateral, i.e., into the foramen, then the root compressed will be the one exiting above. This is known as a far lateral disc herniation and occurs in about 3% to 10% of cases. It is also important to note that while the signs outlined in Table 1 are helpful in the diagnosis and decision making regarding type of treatment, not all of the signs and symptoms associated with a root may be present in an individual, and multiple root signs may even be present.