How Acute Back Pain Becomes Chronic " Part Two "
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 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.
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.
The intervertebral disc and other spinal structures around the spinal segments have been shown to be potential causes of pain. Direct stimulation of the outer layers of the disc has been shown to produce pain in a proportion of patients undergoing operation. The large water attracting molecules break into smaller molecules as the disc ages and repair of this process is slow. The tears and fissures in the annular fibres increase the gradual breakdown and dehydration of the disc structure, with the poor blood supply to the outer disc layers insufficient to prevent the continuing internal disc degeneration.
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.