The phrase ‘slipped disc’ is a term that has fallen out of use in medical circles, for very good reasons. Our understanding of what how a disc functions and what goes wrong has improved with the advent of MRI scans and modern surgical techniques. Up until a decade ago the only way of investigating discs was via standard x-rays. However discs are radio-opaque, showing up as blank spaces in the spine between vertebrae. So from a spine x-ray we can only infer what is going on in a disc rather than is the case when an x-ray confirms a broken bone.
Nowadays our understanding of disc damage is dramatically better. Let me explain a little about the anatomy or architecture of the spine. Discs are found between every two vertebrae (spinal bones), so we have 22 discs from the neck to lower back. The structure and size of each disc varies as you go down the spine, with the largest discs (and most prone to damage) found in the low back just below the waistline.
A lower back or lumbar disc is about 5mm thick. It comprises outer fibrous rings surrounding an inner pulpy semi-liquid core. The fibrous rings are solidly attached to the bottom and top of the vertebrae above and below. This attachment anchors each disc in place, ensuring that the rings which form the outer component of the disc absolutely cannot ‘slip’ forwards, backwards or in any direction. So the answer to the slipped disc question is never! Other things happen to discs but ‘slipping’ is not one of them.
So what does happen? The key to the function of the disc is the inner pulpy centre, called the nucleus fibrosis. The nucleus acts like a soft deformable ball- bearing assisting spinal movement in all directions. The bony vertebrae above and below, coupled with fibrous rings surrounding the nucleus means that the pulpy centre is always held under pressure. Think of the rings as a tube of toothpaste and the nucleus as the paste within.
As we age and cumulatively use our spines more and more, the disc begins to show signs of wear. With age, repeated or awkward movements of the spine the outer rings begin to yield to the internal pressure from the nucleus. The rings develop microscopic damage in the form of tiny cracks or fissures. This is the first stage of disc trauma or disc disease. Gradually or occasionally suddenly, the cracks become bigger until a full tear of one, several or all rings occur.
In either case the pulpy core squeezes into the crack. The tearing process on its own causes local inflammation in the disc which often presents as aching back pain, usually in the immediate area of the spine. The pulpy core pushing into the torn ring further causes pain. Very often this aching pain will settle by itself in a few days. If the tear is big the quality of the pain may be more intense but again the pain will be felt mainly locally.
If the process of crack development occurs very gradually eventually the outer rings begin to buckle a bit, losing height from 5mm to 4 or 3mm. So instead of standing up straight the rings look and behave like deflated car tyres. The loss of disc height is clearly seen on a simple x-ray and infers that some form of disc disease has already occurred. In medical terms this is known as a ‘bulging’ disc and is surprisingly common. Recently it was proposed that 30% of 30 year olds, 40% of 40 year olds, up to 70% of 70 year olds will have this evidence of disc wear on x-ray. The interesting thing is less than 10% of people with this loss of height on x-ray are actually aware of painful symptoms. For many it is gradual onset of back stiffness, slight reduction in flexibility that is noticeable.
Discs are not the only cause of local back pain. Strained or weak muscles, strained ligaments or stiff spinal joints can also become sources of pain. The architectural complexity of the spine is what makes back pain so challenging. It also means that you need to be cautious discussing your back pain with others. Your back pain may not be the same as that of the next person you meet and may require a different approach to get better. If your pain comes from different structures in the back it is like comparing apples and oranges, both are fruit but are entirely different.
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