The discs in between the vertebral bodies are composed of two main parts: the outer layers being the annulus fibrosis and the inner material known as the nucleus pulposus. The annulus is tough and is arranged in layers rather similar to an onion, with the successive layers angled slightly differently to their neighbours to allow good strength in many orientations. The annular layers attach through the bones above and below, anchoring them firmly and meaning a disc cannot in reality slip out. Nerve endings giving both positional and pain information are present in the outermost layers of the annulus.
The nucleus of the disc is encompassed by the inner layers of the annulus and this gives compressive strength to the structure. About two-thirds of the disc is made up of the nucleus and it supports about 75% of the compressive loading. 2.5 times their weight in water can be attracted and held by the large molecules which make up the nucleus, which is 90% water until we get into our 20s, when it starts a slow decline over the next four decades to sixty-five percent. A blood supply is only present in the outer one third of the annulus so the remainder of the annulus and the nucleus must rely on the diffusion of water and nutrients from the vertebrae to remain healthy.
The annulus can be stressed repeatedly by loading and twisting forces which cause microscopic trauma to the fibres and result in annular tears developing. Circumferential tears track around the disc between the layers and radial tears cross the layers from inside to out, with a combination of these tears sometimes developing into larger splits from the inside nuclear material to the exterior. This can permit extrusion of the disc material out of the disc and inflammation or compression of the exiting nerve roots, leading to severe leg pain known as sciatica.
Around eighty to ninety percent of the weight transmitted through the spine goes through the rear third of the disc in the first two decades of life. The stresses gradually move posteriorly as the changes in the spine progress with age and the discs become narrower, throwing stress onto the facet joints, which become enlarged and develop bone outgrowths. The degenerative processes cause progressive narrowing of both the nerve exit canals and the central spinal canal which can cause nerve compression and produce leg and back pain. In older people this is often diagnosed as spinal stenosis which gives leg pain on walking and at times requires operation.
It has been shown experimentally that the spinal structures which include the inter vertebral discs can be sources of pain inputs. In people undergoing operation, direct stimuli to the disc have been indicated to be painful. The proteoglycan macromolecules which make up the internal disc structure and attract water gradually break down into smaller molecules with age and replacement is slow. The steady breakdown and dehydration of the disc is increased by the fissuring and tears which develop in the annulus as the poor internal blood supply is unable to prevent the progress of degeneration.
The poor blood supply through the endplates of the vertebrae may be related to spinal pain problems but the ability to correlate the degenerative changes to the patient's pain symptoms is problematical and cannot usually be connected with any certainty. This makes attribution of a cause and so the probability of therapeutic success difficult.
Biochemically, painful discs have been shown to have lower pH than non painful discs. Reducing the pH in the discs of animals has also been shown to induce pain behaviours and heightened reactions. Experimentally deformed discs in animals have shown increased levels of neuropeptides which gives a clue to their possible functions in the transmission and modulation of pain in the spinal cord. Neuropeptides may be released in response to biochemical forces, stresses and micro trauma, causing the release of inflammatory chemicals and enzymes which break down tissues. All these may add to the increased deterioration of the discs and other spinal structures.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about
Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and
physiotherapists in Edinburgh. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.
