Tough on the outside with a softer, gel-like fluid inside, discs sit between each vertebra. Think of them as car tires on their sides, filled with a thick gel. When your car drives over a bump, the rubber tire 'gives' a little, to absorb the bump. Similarly, each time we move the spine, the discs change shape in relation to the movement. Like so many structures in the body, discs are multifunctional. They are shock absorbers, and they connect and protect vertebral bones. Without discs, bone would touch bone with each movement and eventually grind away.
Also know that the shocks absorbed are usually small and not a problem generally speaking, especially because these discs are quite tough. They do, however, have their limits, just like tires. When the shock is too extreme, something has to give and a tire will blow. In the case of our intervertebral discs, the gel on the inside can burst out (causing a herniated disc) or the outside can protrude (a bulging disc); the discs can also dry out and get thinner (due to disease or sometimes from aging).
The Back and Beyond
Discs are made from collagen; technically speaking, they are fibrocartilage, which means they consist of strong fibers with some elasticity.
Because problems can occur in either the tough outer shell or the gel inner portion, it's good to know what both look like and what your doctor is talking about.
The nucleus pulposus is the gel-like center of the disc designed to absorb shock and provide lubrication. It's mostly made of water. As we age, it can dry up a bit, making the discs thinner and less shock absorbent. There is some evidence that inversion therapy or lumbar traction can help the discs rehydrate, but the long-term benefits have yet to be demonstrated.
Jason Highsmith, MD is a practicing neurosurgeon in Charleston, NC and the author of The Complete Idiot's Guide to Back Pain. Click here for more information about the book.