Intervertebral Disk Disease

Anatomy

The spinal column spans from the skull to the pelvis. There are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae. These vertebrae articulate with the adjacent levels via the intervertebral disk in the front and the facet joints in the back.

The lumbar vertebrae end on the sacrum, which is a fused, V-shaped vertebra and articulates with the pelvis to transfer forces from the trunk to the legs (Figure 1).

The intervertebral disks become progressively larger from the cervical spine down through the lumbar spine. These disks have three basic components: the annular ligaments on the outside, the colloidal gel (nucleus pulposus) on the inside, and the end plates of the vertebrae above and below. The annulus is composed of well-organized concentric ligamentous sheaths (10–20 layers). The nucleus pulposus is a hydrous gel with a few cells and some inflammatory enzymes. It provides shock absorption to the spine, as well as motion. The end plates are at the top and bottom of the vertebrae, where the disk attaches. Since there is no blood supply, the end plate provides nutrition to the disk from the hydrostatic pressure during motion. In the child and adolescent athlete, these endplates are composed of soft growth cartilage and are susceptible to injury.

Figure 1 Portion of the Lumbar Spine Showing the Relative Positions of Intervertebral Disks and Vertebrae

The disks are crescent shaped, with the posterior curved away from the posterior spinal column.

The one exception is the lowest disk at the lumbo-sacral juncture (L5-S1), which is round.

Pathologic Disk Problems

A number of changes occur to the disk over a lifetime. Some are part of a normal process that does not involve injury or pain. As we age, many disks will lose the hydration of the nucleus pulposus, losing some height and appearing darker on magnetic resonance imaging (MRI) studies. These are often normal, benign findings.

Conversely, a number of disk changes may occur that are sometimes associated with pain. However, it is crucial to understand that all the following changes may occur with no associated pain. Making the association between a pathologic change seen on an MRI scan and pain is often difficult. The different pathologies of the disk include degenerative disk disease, annular tears, herniated disks, and disk disease of adolescence. Degenerative disk disease occurs with disk dehydration and loss of the annular ligament strength, leading to instability with excessive motion and stress to the vertebrae above and below. This manifests with irregularities of the end plate and swelling (edema) in the adjacent bone. MRI may show edema in the bone, a dark disk, and tears of the annular ligaments. These changes can be correlated with pain.

...