When Acute Back Pain Becomes Chronic-Part 2

                                                                                         

The vertebral bodies are separated by the discs which consist of two main structures: the inner more flexible nucleus pulposus and the outer, tougher annulus fibrosis. The annulus is in layers with some similarity to an onion, successive layers being arranged at slightly different angles than the previous, giving the whole structure strength in many directions. The vertebral bodies have the outer layers of the annulus running through them, knitting the whole bone and disc complex together and ensuring a disc cannot literally slip out. Positional information and pain inputs are received from the appropriate nerve endings in the outer layers of the annulus.

The inner layers of the annulus surround the internal and more fluid nucleus, allowing the disc to resist compression. The nucleus of a normal disc makes up about 65% of the mass of the disc and supports almost three-quarters of the load. The large molecules which make up the nucleus can attract and hold water at a level of 2.5 times their weight and until we get into our twenties the nucleus is about 90% water. This gradually reduces over the following forty years to around 65% water. Without a blood supply, the inner two-thirds and the nucleus rely for nutrition on the transport of water and other molecules in and out of the disc.

Repeated loading and twisting of the fibres of the annulus fibrosis result in micro trauma to the annulus and tears appear, some of which follow the annulus around rather like a split between the layers of an onion. Others are radial fissures which cross the layers from inside towards the outer part of the disc. If the splitting and radial tears grow together a split may form from the inside to the outside of the disc which is big enough to allow the central nuclear material to protrude from the disc and contact the nearby nerve roots, causing nerve root compression/irritation or sciatica.

In the first twenty years of life 80 to 90% of the weight applied to the spine is transmitted across the back third of the disc. However, as degenerative changes occur and the discs lose height, the axis of stresses moves backwards and loads the facet joints more severely. The facet joints can react to this by increasing in size with arthritic changes and by developing osteophytes. These processes can progress to narrowing the central canal and the nerve exit routes, compressing the central nervous tissues or the nerve roots and causing leg and back pain. Later in life this progresses to spinal stenosis which can give various symptoms and sometimes requires operation.

Potentially painful structures have been shown to be the discs and other spinal structures which make up the vertebral column. During operation direct stimulation of the disc has been shown to cause pain in a proportion of patients. Inside the discs the large water assimilating molecules steadily break down as degeneration proceeds with age, the process being facilitated by the degenerative tears and fissures which occur. Because of the inadequate blood supply the disc is unable to prevent this process.

Chronic spinal lesions may be related to poor blood supply across the endplates but the correlation between spinal pain problems and the degenerative stages is not good. This complicates the ability to relate the changes found on imaging such as MRI scanning and x-ray to the patient's symptoms and so come up with a plausible cause for the pain.

Altered biochemistry in the inter-vertebral discs may correlate with pain experience as discs which are painful have been found to have lower pH than those which are not painful. Heightened pain experience and increased behavioural pain reactions are producible in animal studies by reducing disc pH. In animals whose discs have been experimentally deformed there are increased levels of neuropeptide production, potentially modulating pain in the nervous system. Microscopic tissue trauma, mechanical stresses and changes in tissue chemistry may all heighten production of inflammation chemicals and tissue breakdown enzymes, further increasing the degenerative changes in the discs and spine.

About the author:
Jonathan Blood Smyth is the Superintendent of Physiotherapists at an NHS hospital in the South-West of the UK. He writes articles about back pain, neck pain, and injury management. If you are looking for Winchester physiotherapy visit his website.

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