As a disc progressively degenerates, the ball of fluid at the centre (the nucleus) dries out and load is transferred to the disc walls. In some cases the wall perishes at the points of greatest duress—usually one of the back corners. As the nucleus dries it also loses cohesion and with
excessive twisting and lifting activity it can extrude through fissures in the wall where it is the most broken up. The pressure of the runaway nucleus against the sensitive outer layers of the disc wall can cause severe back pain. Sciatica (leg pain) can also be caused by the displaced nucleus lodging on a nearby spinal nerve root.
Stage 5: The unstable spinal segment
With progressive loss of internal pressure, the disc cannot spring-load its vertebra when the spine bends. With each movement, it goes to shear forward at the problem link, tugging at its own walls as it goes. As it perishes, more strain is taken by the other main structure holding the segment together, the capsular ligaments of the facet joints at the same level. Eventually these ligaments stretch too, leaving the vertebra to wobble about in the column.
In the event of severe arthritic change of the facets, instability can spread from the other side of the segment first. Eventually the disc suffers because stretched facet capsules allow too much movement of the segment. Frank instability of a segment is not a common cause ofback pain.
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