So how exactly does a disc slip? The simple answer is, it doesn’t. Discs are not separate from the spinal bones or vertebrae they sit between; they all form as a single cartilage structure before birth. Bits of the cartilage turn into bone or “calcify”, becoming the vertebrae, while the rest remains as cartilage, forming the discs between the vertebrae. It is the position of the discs between the spinal bones or vertebrae, that gives them the title of “intervertebral discs.” As there was never any separation between bones and discs, there is nothing that can “slip”.
The disc has a tough fibrous outer made up of several layers, this is filled with a softer gel inner. If the outer becomes damaged and weakens, the pressure of the gel inside causes the outer wall to bulge. A similar process can affect the wall of a car tyre, resulting in an “egg” like bulge on the tyres’ side wall, a definite MOT test failure by the way. It is such a bulge on an intervertebral disc that can press on a nerve, restricting the circulation in the area, and leading to inflammation.
The build up of irritating chemicals from the inflammation upsets the nerve leading to pain, and the pressure on the nerve causes the pins and needles or numbness that often affects the arm or leg supplied by the nerve.
Treatment needs to reduce the mechanical strain on the area, so reducing the pressure and the risk of further damage. We also want to encourage better circulation through the area, so that the irritating chemicals can be flushed away, and building materials needed to repair the damaged tissues can be delivered more speedily.
But before we rush into treatment, are we sure that the disc is really the problem? Very often patients report to me that they have had an x-ray, and from this were told that they had a “slipped disc,” this is despite the fact that you can’t see discs on an x-ray! All that can be seen is that the spaces between the vertebrae are reduced; yet a senior spinal surgeon (who spent most of his time rummaging around in peoples’ backs) told me that in his opinion this finding was of no relevance.
Discs, and the nerves that they can compress, are all visible on the super-duper state of the art MRI scans. Disc bulges and squeezed nerves are seen on these scans, but even that doesn’t tell the whole story, because similar pictures can be seen in people who have no symptoms at all.
I recall a patient I treated years ago who developed sciatica (pain radiating down the sciatic nerve in the leg) after a road traffic accident. His insurance company required him to be examined for an independent report by an orthopaedic surgeon, and he let me have a look at the surgeons report. The report went into significant detail about the MRI scan results, explaining how it showed a disc bulge putting pressure on the spinal nerve which the surgeon concluded must be the cause of the patients symptoms, despite the fact that the nerve in question was on the opposite side of the body from the symptoms.
This is why it is so important that treatment is based on an overall assessment of the patient, rather than just on a label for a disease or condition. Assessment needs to be ongoing, with the treatment constantly adapted to how the patient is responding.
Another popular misconception is that discs “crumble”; a more apt description would be to say that they go down or deflate, again rather like a tyre. They lose pressure inside because the inner gel deteriorates, which reduce its’ ability to draw in water. The gel relies on movement of the joint to pump fluid through it, so keeping it fed. Stiffness in the joint leads to starving of the disc, which in turn causes it to deteriorate. So make sure you keep your spine mobile if you want to stop your disc from “slipping” or “crumbling”!