The intervertebral discs are composed of an anulus fibrosus and nucleus pulposus.

• The anulus fibrosis consists of 10-20 concentric sheets of collagen fibres, called lamellae, tightly packed together. The orientation of the fibres alternates in successive lamellae but their angle with respect to the verticle remains roughly the same, and measures about 65 degrees. These lamellae are stiff but are still pliable enough that they can defom and allow bending movements. The anulus fibrosus encloses the nucleus pulposus.
• The nucleus pulposus contains a hydrated gel-like matter located in the centre of the disc. The nucleus pulposus consists largely of proteoglycans molecules which are important because they attract and retain water. The amount of water in the nucleus varies during the day depending on activity.

During normal daily activities, the intervertebral discs are subjected a variety of loads whereby water is "squeezed" out of the discs and they lose height, much of which occurs in the first hour of the day. Following unloading during the night, the water is re-imbibed, disc height is restored and the mechanical properties are restored. This restoration of the discs mechanical properties naturally requires the avoidance of structural damage which can be caused by the spine not being supple and elastic and not being free from abnormal curvatures and patterns of pre-rotated vertebrae. As well as the mechanical difficulties experienced with this disc degeneration, as mentioned previously, there may also be damaging effects on the nutrition of the disc and the proteoglycans of the nucleus pulposus which will prevent the disc restoring water content and disc height.

Disc degeneration will involve changes in the disc composition, structure and function and may eventually result in disc prolapse. A slipped disc is another common term for a prolapse but can be very misleading for the patient as they may envisage a disc which can slide about. In reality, it involves a displacement of the nucleus pulposus through a tear in the anulus fibrosus beyond the normal perimeter of the disc, usually into the vertebral canal or intervertebral foramen and can compromise the spinal nerve or its roots. This can cause considerable pain, numbness or weakness and/or damage to the nerve and this most frequently occurs in the lumbar region.

Mechanically, the best way to improve the situation is to unload excessive stresses from the disc to allow healing processes to take palce and mechanical function to be restored as much as possible. This should help to reduce the chances of the same problem re-occuring in the future. If the extruded fragment of nucleus pulposus is not removed surgically (this is only needed in extreme situations), it can shink markedly over time and may even disappear completely when viewed on an MRI image.