The Change From Acute To Chronic Back Pain ” Part Two

by Jonathan Blood Smyth

The inter-vertebral discs are made up of two main segments: the outer shell called the annulus fibrosis and the inner material called the nucleus pulposus. The outer lining is tough and has many layers like those of an onion with the different layers being oriented at differing angles to give strength in many directions. The layers of the annulus pass through the vertebrae above and below them, binding them all strongly together and meaning that a disc cannot actually slip out. The outer layers of the annulus are supplied with nociceptors and proprioceptive nerves, an indication they can send both pain and positional information back up to the nervous system.

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.

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.

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.

Pain problems in the intervertebral discs may also involve biochemical and other factors and a lower pH has been found in painful as compared to non painful discs. In animal studies reduction in the pH of the discs heightens pain reactions and increases the pain behaviour of the creatures. Increased neuropeptide levels have been produced in the experimentally deformed discs of animals and could be involved in modulation and transmission of pain in the central nervous system. Mechanical stresses, micro-trauma and biochemical changes may increase production of inflammatory chemicals and enzymes which can breakdown tissues. These factors may all increase the disc and other spinal structure changes.

About the Author:

Jonathan Blood Smyth is the Superintendent of Physiotherapy 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 physiotherapists in Bolton visit his website.