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.
The annulus can be stressed repeatedly by loading and twisting forces which cause microscopic trauma to the fibres and result in annular tears developing. Circumferential tears track around the disc between the layers and radial tears cross the layers from inside to out, with a combination of these tears sometimes developing into larger splits from the inside nuclear material to the exterior. This can permit extrusion of the disc material out of the disc and inflammation or compression of the exiting nerve roots, leading to severe leg pain known as 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.
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.
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