Discs are disc shaped gel filled sacks which are woven in a fibrous nest of collagen. This gel provides discs with their most important quality which is a high resistance to compression. This quality comes from the gel’s ability to bind water; under normal conditions water is incompressible. This is their most important function because the discs are orientated in between the vertebrae and are exposed to high levels of compression. Levels of compression increase at lower levels of the spine and as such the biggest discs are in the lower back.
Although discs are very good at resisting compression they are not as good at resisting rotation, especially when they are fully loaded. Due to this discrepancy the classic injury mechanism of a disc is in a loaded position of forward bending with added rotation. A real life example of this would be weeding the garden from a standing position or lifting a heavy object from a forward bent and off centre position.
The discs themselves provide the spine with a great degree of flexibility which is at its height in the neck and lower back. In the thoracic spine ribs limit the range of movement. There are also twinned joints between all the vertebrae called facet joints. The shape of these joints are what dictate the particular ranges of movement at all spinal levels.
Discs themselves are not particularly pain sensitive when undamaged but after they are injured extra blood vessels are formed to deliver blood which help heal the damage. These blood vessels bring with them a nerve supply and thus create increased sensation in the disc. Repeated injuries can create an increasing level of sensitivity over time meaning that the level of pain experienced from an injury may not always reflect the degree of damage.
In addition to increased sensitivity discs will undergo a decrease in height with subsequent injury. This is because when the original disc material is damaged it is replaced with scar tissue. This scar tissue does not have the special ability to absorb great quantities of water like the original tissue which causes the size of the disc to decrease slightly. Over time this can affect an individual's overall height but would rarely be significant enough to be noticed by itself.
Disc injuries are most common in the lower back due to the high level of forces regularly passing through this region. Muscular compensatory contraction patterns for disc injuries may actually cause severe symptoms over time due to overuse related changes such as increased stiffness, lack of blood supply and loss of original function.
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