Fig 1: Healthy Microtome Section
(larger view)

Fig 2: X-ray of Degenerative Disc Disease
(larger view)

The vertebral disc in the spine is an interesting and unique structure. Its primary purpose is to act as a shock absorber between adjacent vertebrae. Spinal discs also act as ligaments that hold the vertebrae of the spine together and as cartilaginous joints that allow for slight mobility in the spine.

There are a total of twenty-three vertebral discs in the spinal column. Specific problems with any of these discs may prompt different symptoms, including back pain, neck pain, and sciatica.

Spinal Disc Construction

Discs are actually composed of two parts: a tough outer portion and a soft inner core, and the configuration has been likened to that of a jelly doughnut (see a healthy disc with Figure 1).

  • The outer portion of the disc (annulus fibrosus) is the tough circular exterior composed of concentric sheets of collagen fibers (lamellae) that surround the inner core.
  • The inner core (nucleus pulposus) contains a loose network of fibers suspended in a mucoprotein gel.
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The annular fibers hydraulically seal the gelatinous nucleus and evenly distribute pressure and force imposed on the structure.

The outer portion and inner core of the spinal disc fit together like two concentric cylinders and are interconnected by cartilagenous end-plates.

At birth, 80 percent of the disc is composed of water. In order for the disc to function properly, it must be well hydrated. The nucleus pulposus is the major carrier of the body's axial load and relies on its water-based contents to maintain strength and pliability.

Disc Degeneration

Over time, spinal discs dehydrate and become stiffer, causing the disc to be less able to adjust to compression. While this is a natural aging process, as the disc degenerates in some individuals, it can become painful.

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The most likely reason for this is that the degeneration can produce micromotion instability and the inflammatory proteins (the soft inner core of the disc) probably leak out of the disc space and inflame the well-innervated structures next to the disc (e.g. nerve roots). Sometimes a twisting injury damages the disc and starts a cascade of events that leads to degeneration (see a degenerative disc x-ray with Figure 2).

The spinal disc itself has very few nerve endings and no blood supply. Without a blood supply the disc does not have a way to repair itself, and pain created by the damaged disc can last for years. In general, as we age there are less inflammatory proteins in the disc space and discogenic pain rarely occurs after 60 years of age.