I propose that a simple back pain develops when an intervertebral disc (the fibrous pillow between the vertebrae) loses water and stiffens. This can be caused by several factors, not least small-scale injury—
either across or through the length of the spine—and long-term compression. Then one of two things can happen for the problem to get worse: you can develop more serious trouble from the front compartment of the spine as the disc breaks down further. Or you can develop trouble from the back compartment as strain translates across the spine to the facet joints. Worse still, you can get pain from both compartments at the same time. Finally, the wholesale destruction of both compartments can cause the vertebrae to slip around, in what is called segmental instability.
front back compartment stiff compartment
prolapsed 'slipped' disc facetjoint arthropathy
acute locked back unstable spinal segment
Diagram 1 A spinal segment can break down via the front compartment as the disc stiffens, or through the facets of the back compartment. As the two structures weaken the segment may become 'unstable'.
The sequential disorders in the route of breakdown are as follows: Stage 1: A stiff spinal segment
An intervertebral disc ceases to be a buoyant pillow and becomes like a hard non-compliant washer between two vertebrae. The vertebra on top loses mobility and the segment becomes like a stiff link in the spinal chain. Because the disc lacks movement it cannot suck and blow to feed itself, so it shrinks all the more and further retires from activity. Eventually the flattening can be picked up on X-ray, but well before this point the condition can be painful—in what I believe is the most common spinal disorder.
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