Slipped Disk

Anatomy

The spinal column descends from the skull to the pelvis with 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae. The articulation of the adjacent-level vertebrae occurs via the intervertebral disk in the front and the facet joints in the back (Figure 1). The lumbar vertebrae end on the sacrum, which is a fused V-shaped vertebra that [Page 1354]articulates with the pelvis to transfer forces from the trunk to the legs.

Figure 1 Lumbar Vertebrae (lateral view) and Single Vertebra (top view)

Source: Illustration by Michael d'Hemecourt.

The disks become progressively larger from the cervical spine down through the lumbar spine. The disks provide shock absorption to the spine as well as motion. These disks are composed of three basic components: the annular ligaments on the outside (annulus), 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 ligamentous sheaths (1020 layers; Figure 2). The nucleus pulposus is a hydrous gel with few cells and some inflammatory enzymes. The end plates are at the top and bottom of the vertebrae, where the disk attaches. Since there is no blood supply to the disk, the end plate provides the nutrition to the disk by way of hydrostatic pressure during motion. In the child and adolescent athletes, these end plates are composed of soft growth cartilage and are susceptible to injury.

The disks are round and crescent shaped, with the posterior curved away from the posterior spinal column. The one exception is the lowest disk at the lumbar-sacral juncture (L5-S1), which is round. In the adult, the disk has the weakest ligamentous constraints on the posterolateral side, where the nerve roots exit. As such, a rupture here will affect the nerve. Conversely, the child and young adolescent may rupture into the soft end plate growth cartilage.

Etiology

A slipped disk represents a herniation of the nucleus through a tear in the outer annular ligaments. When a small isolated tear occurs, the nucleus stays in place except for mild bulging. This annular tear will appear as an intense white line on some magnetic resonance imaging (MRI) scans. This is called a high-intensity zone (HIZ) and has been correlated with pain.

...