The Development Of Chronic Back Pain ” Part Two
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 more fluid nucleus part of the disc is surrounded by the inner layers of the annulus which support it and allow strength under compression. A normal disc is made up of about sixty-five percent nucleus which is able to sustain three-quarters of the disc load. The nucleus contains macromolecules which can attract and hold two and a half times their weight in water, the nucleus being made up of 90% water until we approach our thirties and it declines to around 65% four decades later. The outer third of the disc annulus receives a blood supply while the rest does not and must depend on the nutrients and water diffusing across from the vertebrae above and below for nutrition.
If the fibres of the annulus fibrosis are subjected to repetitive forces of twisting and loading then micro-trauma can result to the annulus which occurs in two patterns. Tears can run along the successive layers of the annulus (called circumferential tears) or across the annular layers (called radial tears), with larger and more significant tears developing when these two types coincide in one area. This can allow a much bigger tear to appear which allows central nuclear material to protrude right out of the disc and to compress or inflame the nearby nerve roots, causing 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.
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.
The poor nutrition which passes through the vertebral endplates may be partially responsible for chronic spinal lesions, but there is a poor connection between degenerative changes and vertebral pain. This makes the interpretation of x-rays and MRI scans difficult in terms of attributing a cause to a patient’s pain which correlates with the investigation findings.
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.
