The Intervertebral Disc

Disc disease, degenerated discs, slipped discs, herniated discs are common terms often related to back pain, both the lower back and the neck. What do these terms mean? How do these things happen? What can one do to help or prevent these painful events from occurring? These questions and many others will be discussed in this and a following article. Hang on.

First some anatomy to get us on the same page: The spinal column is made up of 24 bones called vertebrae. Each vertebra has six joints; four at the back of the bones that allow and control spinal movement. These are called facet joints and are aligned vertically, parallel to the direction of the spine. The other two joints are at the top and bottom of the vertebrae themselves. These joints are horizontal joints in orientation and weight-bearing in function. Between each pair of the vertebrae is an intervertebral disc, except between the top two in the neck. The discs become progressively smaller as you go up the spine. They also change shape as they go up, simply because the bones also change shape.

The most problematic discs are the lower back ones, so I'll focus mostly on them. The typical lumbar disc is almost circular in shape, but looks more like a flat car tire or mushroom head. It's flat across the back when you look down from the top.

A disc is made up of two things: the soft central area is called the nucleus, and the tough outer cartilage margins are called the annulus. The nucleus is roughly spherical in shape and is made of a toothpaste-like material. The nucleus works like a ball bearing, allowing the bones to flex and extend around its shape, much like exercising over a gym ball in Pilates. The weight of the body causes a considerable amount of pressure to be built up in the nucleus. One of the things that builds the most pressure in a disc is when you sit, especially in poorly supportive furniture, or when you slouch. Surprisingly, the pressure in the nucleus is three times greater in sitting than it is with standing. Doesn't that speak volumes when you think of today's lifestyle?

The annulus is wrapped in layers around the nucleus to contain its pressure. These layers are somewhat like the layers of an onion. The fibers of one layer are at right angles to the next layer. These layers have to be tough and non-yielding; otherwise the soft nucleus would lose its shape and spill all over the place. In fact this is indeed what does happen if the annulus breaks down. This incident is known variously as a slipped disc, a herniated disc, a disc protrusion, or a disc bulge. These terms often depend on the degree of nuclear movement, which depend on the degree that the annular cartilage breaks down. More on all this later.

Before the age of twenty-five, the disc has a blood supply. Two blood vessels enter the disc from the back and these vessels angle towards the centre and supply the nucleus with oxygen and nutrients. These blood vessels enter the disc from the posterolateral direction or 4:00 and 8:00 positions, if you can imagine the top of the disc like a clock with the flat portion at 6:00. Coincidentally, these positions correspond with the areas of the disc that are closest to the nerves that exit the spine.

After the age of twenty-five or so, these two blood vessels dry up. That leads to two problems: Firstly, without a blood supply, the discs only get the sustenance they need from osmosis, which is greatly aided by spinal movement. Flexing, extending and rotating a disc through exercise pumps the disc and draws in oxygen and nutrients. Therefore, I'm sure you can readily see how a sedentary lifestyle prematurely ages and breaks down the discs.

The second factor in the drying up of these blood vessels is the fact that the vessels leave a channel through the annulus to the nucleus thereby creating a weakness in the disc. In fact, almost all disc herniations and bulges follow this inherent weakness as the nucleus forces its way through the annulus to the posterolateral edge of the disc. It is at this edge, as we pointed out before, that the spinal nerves lay. In the case of the lumbar spine, the most commonly herniated discs are the lower three which also correspond to the pathways of the sciatic nerve.

At some point in the near future, I will write another article about the intervertebral disc, Part 2, if you like. Thanks for reading.

Author Contact Info: David Phillips D.C.
http://miltonchiropractic.chiroweb.com/