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.
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