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Development of spinal segmental instability.

In health, the fluid filled nucleus pulposus is contained within the intervertebral disc by a competent annulus fibrosis. Either through age or injury, the annulus fibrosis can develop fissures (cracks) through which part of the nucleus pulposus can pass. This has the affect of decompressing the disc (like slightly letting down a car tyre). As a result of this, movements between the vertebrae separated by the disc become less uniform and the centre of axis of movement changes markedly. This has the affect of making the disc unstable or wobbly.

One affect of the change in mechanics of the movement can be that the supporting ligaments become lax or stretched and therefore do not provide adequate support of the vertebrae, making the instability worse.

The affect is magnified further because patients with low back pain of just a few weeks duration often develop weakness of the segmental, stabilising muscles. These will therefore fail to give adequate support to the low back.

There are several potential affects of these changes.

1. Pain may be felt from the disc itself.

In the outer margins of the annulus fibrosis are pain receptors. The change in mechanics of the disc may mean that the fibres of the outer disc stretch more easily on certain movements or may be stretched by the soft nucleus polposus passing into the outer fibres causing a ballooning affect. This stretch may be perceived as pain.

2. Pain is perceived from nerve tissue being irritated from the disc.

With the disc being let down like a car tyre, some bulging of the disc may occur. If this bulge is backwards into the spinal canal, then this can cause some compression and/or irritation of the spinal nerves which may be perceived as pain, loss of sensation and/or power in the leg.

3. The disc contributes to marked instability.

Instability may be perceived as the back feeling weak on certain movements which may be painful. There may be a background pain which is made worse by certain repetitive movements, prolonged sitting or standing and occasionally with coughing or sneezing and there may be times when the back “goes” or “gives”, when pain is severe and immobilising for varying lengths of time. During these episodes, it may be common for the more powerful spinal muscles that overlie the weak segmental, stabilising muscles, to go into spasm. As well as causing stiffness of the lower spine, spasm of these powerful muscles can put extra compressive force through the painful lumbar discs, causing more local pain.

4. The facet joints may become additional sources of pain.

When a lumbar segment becomes unstable and the movement between vertebrae becomes less uniform, the surfaces of the facet joints fail to glide upon each other smoothly during movement and over a period of time they will begin to change shape to accommodate the change in mechanical forces. As the disc has become narrower, more weight is taken through the facet joints, a function for which they were not designed. The combination of these factors, can contribute to development of degenerative changes within the facet joints (osteoarthrosis) which may (but not always) cause these joints to become painful.

Illustration showing the changes associated with osteoarthritis of the facet joints. © Louise Carrier 2007.

5. Pain is perceived from nerve tissue being irritated from the facet joint.

The changing shape of the facet joints, particularly the fact that they may become more bulky, may mean that they impress forward into the spinal canal. As they lie so close to the exiting nerve roots, this may contribute to pain, loss of sensation and power in the lower leg.

Illustration showing the cumulative effect of disc bulging and enlarged facet joints. © Louise Carrier 2007.

6. Development of a degenerative spondylolisthesis

The combination of all the above responses may lead to the spinal segment becoming lax and one vertebra may slip (usually forward) with respect to the lower one. This may be associated with pain from the disc, facet joints or from compression and irritation of the nerve roots due to all the reasons above.

7. Development of spinal canal stenosis.

The combination of the disc bulging back into the spinal canal and the enlarged facet joints projecting into the spinal canal from either side, may mean that in time, the spinal canal becomes significantly narrower. As a result, particularly when standing and walking, i.e. when upright (when the spinal canal is narrowed further) pain, aching, loss of sensation or power in the lower legs may be experienced. On sitting, or bending (when the spinal canal opens out a little) these symptoms may diminish.

Illustration showing spinal canal stenosis. © Louise Carrier 2007.