The Development Of Chronic Back Pain ” Part Three

In some cases of back or leg pain corticosteroid injections may provide some relief and the mechanism may be that the steroid reduces any inflammatory process present in the tissues. Steroids inhibits inflammatory chemicals some of which have been found in elevated levels in operatively removed disc material from humans. Such inflammatory mediators may promote disc degeneration and make the fibres of the annulus more sensitive to stresses. Other chemicals may add to disc related pain problems by making pain nerves more sensitised, reducing synthesis of the water holding proteoglycan chemicals in discs and by increasing the degrading effects on the disc matrix.

The nerve structures which are part of the lumbar motion segment can be sensitised and so become hyper reactive, leading to mechanical stresses which normally would be painless becoming painful. Nerve root or radicular pain is the pain caused by compression, irritation or inflammation of one of the spinal nerves at or close to the area they emerge from the spinal cord. It is not clear exactly what is the pathology underlying severe sciatica, although hypotheses include compression plus dysfunction of the axons, ischaemia, inflammation and biochemical factors. Spinal nerve roots may be more susceptible to compression injuries and to swelling formation inside their structure than other nerves.

If compression increases the tendency of the nerve to allow fluids across its membranes then oedema can result as too much fluid enters the nerve. This increased internal nerve pressure may block blood flow in the capillaries and lead to fibrosis developing in the nerve. Spinal nerve roots achieve around 60% of their nutrition from the cerebrospinal fluid and any fibrosis around the nerves can reduce this, making the nerves hyper-responsive and so more vulnerable to compression. Loss of blood throughput or slowing of the flow may induce nerve changes which generate pain rather than the loss of feeling followed by loss of muscle control which follows compression of a peripheral nerve.

Quick development of compromise to the nerve and its blood supply is more likely to produce the onset of nerve root symptoms than a slow development. This may explain how many older people appear to have compromised nerve exits but do not suffer symptoms. An inflammatory reaction ensues if disc material is placed in the epidural space and disc material may cause nerve injury when in contact with it. Other possibilities are that tumour necrosis factor (TNF) may be involved as injection of this agent into the nerves causes similar changes that occur in nerve root compromise. An inflammatory or auto-immune response to the normally inaccessible nuclear disc material has also been proposed. Many factors may be responsible overall.

The facet joints are synovial joints, like our finger joints, and are formed from articular processes on an upper and a lower vertebra. Facet joints react to injury and damage in the same way as our other joints react, with pain, stiffness, loss of movement, muscle spasm and eventually degeneration. As the discs degenerate the facet joints follow by becoming osteoarthritic. The fibrous capsules which surround the facet joints are rich in nerve endings and injections into the joints have been shown to refer pain into the arms, upper back and legs. Facet joints are capable of generating pain and have been shown experimentally to be involved in significant percentages of patients with neck, thoracic and lumbar pain.

The sacroiliac joint, a complex joint between the sacrum and the ilium, is known to refer pain to areas over the joint, in the buttock, the lower lumbar spine, the leg and the groin. Experimental work with patients with chronic back pain has shown a variable involvement of the sacroiliac joint in from 2 to 30% of cases. Muscles also have pain receptors which are sensitive to pressure and stretching amongst other stimuli. If they are overloaded either in one instance or repeatedly then injury can occur with pain, shortening and increased tension.

A described muscle pain problem is myofascial pain syndrome, in which muscles are regularly used incorrectly or have been damaged develop tightness and trigger points. Trigger points can be palpated in the muscles as tender and firm tissue areas which then refer pain out into well known nearby regions.