These are flexible plates of fibrocartilage that connect any two adjacent vertebrae in the spine, between them accounting for a fifth to a quarter of the length of the spinal column. Each disc has two main parts:
In the centre of the disk is the nucleus pulposus, a gelatinous substance; and
Surrounding the centre is a ring of very strong fibrocartilage -the annulus fibrosus – whose outer edges are made of collagen, making it much stronger than most other ligaments in the body. Apart from connecting the vertebrae in a flexible manner, the
discs also serve as a series of shock absorbers that help protect the spinal cord and the brain from the effects of impact resulting from the body’s movements. For example, were it not for the discs, the impact produced just by walking would send shock waves directly through a more or less rigid spine directly to the brain where damage would eventually result. Two key points to note about intervertebral discs:
The younger you are, the more effective they are as shock absorbers. At birth, the nucleus pulposus is extremely elastic, but as you grow older, this jelly-like centre becomes harder, some of its gelatinous substance having been replaced by cartilage.
Apart from any natural deterioration that accompanies ageing, the discs can also become damaged by accident or disease. One all too common occurrence is a prolapsed intervertebral disc (often simply referred to as PID, or a ‘slipped disc’), a condition in which the pulpy inner material of a disc protrudes through the fibrous outer ring. When this happens the protruding material can exert pressure on adjoining nerve roots and ligaments, and should this pressure affect the roots of the sciatic nerves, then one or more of the typical sciatic pains may follow. As far as what causes a disc to prolapse – incidentally, this word simply means the falling down or slipping out of place of an organ or a part of the body – this is usually the result of one of two scenarios:
As part of the natural wear and tear that marks ageing, the outer ring of a disc will gradually have become weaker and weaker until one day it is so weak that the smallest amount of extra pressure upon it allows part of the gelatinous centre to pass through it. The final precipitating incident that causes a disc’s prolapse may be quite minor – such as an awkward bending or twisting movement, or even a sneeze or a cough – and would not have had any repercussions were the disc’s outer ring not already very weak. In many ways, this can be described as an accident waiting to happen. If a disc is so weak that it prolapses because of a sneeze, then it would have done so sooner or later.
Alternatively, a disc may prolapse because it has been subjected to an unusually great amount of stress, such as can happen during a fall or a road accident. Naturally, the condition of the disc will to some extent dictate whether it prolapses or not in given circumstances, but any disc, no matter how healthy and resilient, can fail when subjected to enough force.
While a prolapsed disc is one of the major causes of sciatica, there are also several others, including:
A disc that’s become distorted or bulges. Without actually prolapsing, an intervertebral disc may just change shape sufficiently, usually because it’s affected by nearby muscles in spasm, so that it’s flattened, part of its squeezed out section creating pressure on the nerves.
Various rheumatic diseases can attack the spinal joints, causing them to swell, so putting pressure on the nerves.
Osteoporosis – a disorder that causes the loss of bony tissue, resulting in bones that are brittle and liable to fracture – can damage the vertebrae, one possible consequence of this damage being pressure upon the nerves.
Ankylosing spondylitis – the first word describes a process of fusion of the bones across a joint space and the second means ‘stiffening’ – is a disorder in which the joints of the spine become inflamed. As the initial inflammation diminishes and healing takes place, extra bone may grow out from the sides of the vertebrae and can fuse these together, leading to a spine that’s stiffened. Sciatica is a common symptom of the early stages of ankylosing spondylitis.
Spinal stenosis. This is a condition in which the spinal canal has become narrowed.
As you can see from the above, the possible root-causes of sciatica are many and varied. While the symptomatic pain is usually due to a fairly straightforward problem that can generally be resolved through some simple remedies and precautions, there is always a possibility that a more serious disorder is responsible. For that reason, it is essential that sciatic pain that persists and fails to respond rapidly to rest and ordinary analgesics be investigated promptly and its cause fully established. Naturally, the way to find out what is causing your sciatica is to consult your doctor
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