Discs are highly modified joints which lie between each of the vertebrae in the spine. Not only do they act as a joint to allow movement between adjacent vertebrae but they also have important shock-absorbing properties. Discs are oval structures and a few millimetres in height and their size depends on their level in the spine. The discs in the neck (the cervical spine) are significantly smaller than in the lower back (lumbar spine).
Discs are composed of two main parts. The first of these is the annulus which is a tough fibrous capsule of concentric fibres running around the circumference of the disc. The second part is the nucleus pulposus. This is a far softer material which provides the discs with its shock-absorbing properties.
Unfortunately, discs, as with other joints in our body, are prone to a degree of wear and tear as we get older. This is partly a function of the amount of strain we expose them to as a consequence of physical activities we undertake. It is also partially genetically determined and undoubtedly some people’s discs are more resilient than others. As a consequence, disc prolapse do tend to run in families. The amount of wear in any given disc is also dependent upon the amount of movement possible in that part of the spine. Therefore, the more mobile parts of the spine in the neck and lower back are where discs tend to wear the most.
There are a sequence of changes which effect discs as they wear. The first of these changes is simple dehydration and this is usually quite evident on a scan. This is usually entirely asymptomatic.
However, dehydration can make the capsule of the disc (the annulus) more brittle. This can be followed by small tears developing in the annulus and this can be a cause for sudden acute lower back pain. The back pain itself is largely a function of reflex muscle spasm that can accompany a small annular tear. A simple annular tear without a disc prolapse will usually be a self-limiting problem with spontaneous resolution of the muscle spasm in due course and is best treated with rest as required, followed by physiotherapy.
On occasion, an annular tear can open up with a defect in the capsule of the discs such that some of the contents of the disc (the nucleus pulposus) can extrude through the tear. It is this situation that is referred to as a disc prolapse or a disc herniation or a slipped disc.
The disc material that comes out of the confines of the disc space can be very small and essentially asymptomatic. However, if a sufficiently large amount of disc material extrudes, it can exert pressure on the adjacent nerves in the spine, resulting in sciatica or brachialgia (hyperlinks).
Disc prolapses can then spontaneously regress with retraction of the disc material back towards the disc space. However, on occasion when this does not happen, on-going nerve irritation can be an indication for surgery.