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Discs Don’t Slip

There is nothing slippery about a disc. Discs are living adaptable force transducers that are firmly connected to the bones in your back and supported by really powerful ligaments. Diffs slip, soap slips, disc definitely don’t slip.

- G Lorimer Mosely and David S Butler

Discs have a bad reputation. We associate them with the ideas of slipping, bulging, herniating, protruding, extruding and degenerating. But we have twenty three of them in our spines- surely they don’t all do this? And even if they do bulge, protrude or extrude this does not always mean that we have pain as a result. In scans of 98 asymptomatic subjects 52 percent had a bulge at at least one level, 27 percent had a protrusion, and 1 percent had an extrusion. This lead the researchers to conclude that the discovery by MRI of bulges or protrusions in people with low back pain may frequently be coincidental.

Our discs are amazing structures. They act as joints between our vertebrae, they separate and connect the many bones of our spine. They help our body absorb force and transfer it, they create mobility and hold us upright. They are more efficient than bone because they are not constantly building and breaking down in the way that our bones are. They don’t need the same fuel as our highly vascularised bones do either. Instead of giving support through a strong structure, they give us support through their ability to stretch and spread load.

Discs are made up of connective tissue (or fibrocartilage) that contains collagen and proteoglycans (or glucosaminoglycans); these are water binding proteins that bind 10,000 times their weight in water. The outside of the disc is called the annulus fibrosus. Which is made up several layers of strong and stiff collagen designed to withstand compressive forces. Here is a gorgeous image from the University of Exeter of a polarised light microscope slide of the fibres of the annulus.

The centre of the disk holds the nucleus pulposus which has much looser collagen fibres suspended in a mucoprotein gel. Imagine that each disk is like a water bed or an inner tube. When we push down on the centre the sides stretch or expand and create an evenly distributed tensile force. Here is a much less beautiful (but informative) image for you (thank you Wikipedia):

Under load the pressure moves from the nucleus pulposus to the annulus to the endplates then to the vertebrae. This serves to spread load over time and slows any impact of weight through the body. Our endplates are the interface between the disc and the vertebrae. They are made from hyaline and fibrocartilage and serve as the medium through which blood and nutrients flow from capillaries in the bone to cells in the discs. This is important because intervertebral discs do not receive their own blood supply. Nutrients get to disc cells through diffusion. When the disc is compressed fluid moves out into the endplate and is returned during decompression.