Intervertebral discs are a vital component of your spine providing shock absorptive properties whilst maintaining a healthy gap between the vertebrae to prevent overloading of the facet joints. Their role within the spine makes them inherently at risk of injury and degeneration and subsequently account for a large proportion of low back pain either directly ( e.g disc bulge) or indirectly (causing facet joint overload).
Unfortunately their reputations appear to cast impeding doom on to any would be victim and this is more often than not far from the truth so hopefully we can dispell alot of these misconceptions for you.
There are 33 bones in our spines and between each of the 24 moveable vertebrae sits an intervertebral disc (see above). The discs are situated on the large body of the vertebra at the front with the spinal cord sited in the bony canal just behind it. The rear of the spinal canal is made up by the bony complex that creates the moveable facet joints and provides major attachment points for muscles and other soft tissue.
The discs consist of three main sections, an outer Annulus, an inner Nuclueus Pulposus and two endplates, one on the top and one on the bottom. They are largest in size at the base of the spine, getting smaller as you ascend. This trend is also seen within the vertebral bones themselves and both are a reflection of the much higher higher loads experienced within the lumbar spine.
The outer annulus as you can see above, is made up of strong, inelastic collagen rings, around 15 to 20 of them. Each of these layers is adjoined to its neighbour at an angle of around 120 degrees with an elastic ‘coating‘ between them. These structural characteristics allow the annulus to withstand very high levels of tension however the flip side is that the resistance to compression is poor. When we consider that one of the discs main functions is to maintain the space between the vertebrae whilst we work against gravity, i.e. anything upright, this is a bit of a floor, so how do we overcome it?
In the centre of the disc is a viscous gel called the Nucleus Pulposus. In the same way that the annulus exerts a uniform inward pressure, the nucleus exerts a similar but opposite pressure outwards. The ‘inflation’ needed to provide this pressure is derived from the metabolising, that is the breaking down, of nutrients which are supplied to the nucleus. This balloon like facility helps to provide the disc with the high resistance to compression that is absent from the annulus.
There are two cartilagenous endplates that sit on the top and bottom of the individual discs. They are joined at the inner edge of the annulus to the disc and keep the annulus and nucleus contents separate from the vertebral body. In adulthood they become vital in the delivery of nutrients to the disc however this is discussed further below.
The 5 pictures below have been created in order to give a clear impression of what happens to cause disc related pain. Picture 1 is a healthy disc. There is equilibrium between the inward and outward pressures and thus the functional characteristics of the disc perform as they should. In picture 2 however we begin to see some annular delamination. This is when ‘cracks‘ or ‘splits‘ occur between the layers of the annulus. These are thought to occur due to adverse pressures being applied through the annulus when the disc is not functioning at its full potential.
The disc goes through a constant repair/damage cylcle as it responds to the daily forces applied to it and for this cycle to continue effectively the nucleus must have a sufficient supply of nutrients to metabolise and be able to remove the subsequent waste. If this exchange system is starved then the nuclues begins to loose its ‘inflated’ characteristics making the annulus more prone to compressional forces which as we know it does not cope well with. The consequence of this is a rise in the risk of annular delamination.
In childhood the discs have a direct blood supply which allows the exchange process to occur effectively however we loose this supply when we become fully mature (18-20 yrs). We then rely upon the end plates to employ an exchange system through a series of small channels contained within their structure. Whilst this exchange mechanism operates fully the disc maintains its health however it appears to be more prone in this setup to failure. Current research which is now being expanded on an international scale estimates that up to three quarters of all degenerate related disc problems are genetically linked. This has huge implications in our long term understanding and treatment regimes, however why this occurs is still a long way off being understood.
When annular tears occur within the inner two thirds you will normally have no pain as there is no direct nerve supply. In diagram 3 (above) these tears are begining to occur further out. This generates a mechanically disadvantaged area that during forceful activity is prone to nucleus migration which can begin to cause the posterior (back) part of the disc to bulge. It is important to highlight the fact that a bulging disc in isolation will not necesarily give you pain unless the annus is directly irritated in its outer third. Disc bulges are in fact extremely common and a large proportion of the general population, if scanned, would probably have a small one somewhere. You often find that a mild diffuse aching may occur as a symptom of this but would probably be so sporadic, mild and self limiting that it would not make you seek medical intervention.
It is not until we reach diagram 4 below that you may get pain, especially nerve pain. Here, although the nucleus is still contained within the walls of the annulus, the mechanical insufficiency is such that the bulge has increased in size and is now pushing on the nerves directly. In this example the bulge is to the left hand side which would cause left leg symptoms however it can be on either side, or in some cases in the middle. The latter can give bilateral (both) symptoms and in rare cases, depending on the spinal level affected can cause Cauda Equina symptoms. This is where the sudden loss of bowel and bladder function occurs and in these extreme cases inmediate medical intervention should be sought.
In the fifth and final diagram we see the most severe kind of disc related injury and this is where the annulus has been completely breached so that the nucleus is exposed. The nucleus has an very low PH value (acidic) so its exposure to soft tissue will cause a significant inflammatory response. In some cases little bits of nucleus may even break off (sequestration) in to the spinal canal and these must be removed if the body does not absorb them independently .
Slipped Disc?
Hopefully you will now beable to appreciate how unlikely this is. The annulus is very firmly attatched to the vertebra and is not going anywhere! The pain and symtpoms we get directly from disc related pain are much more likely to be from bulging and soft tissue/neural compression. So anyone who offers to ‘put you disc back in’ may be worth deleting from your therapeutic contact list!
There external links below are worth viewing for alternative descriptions of the above process. It is also advisable to read the disc degeneration section as this is the underlying cause of many prolapse/herniation events and contains numerous links to other good sites relating to this.
