Abstract
Introduction : We have shown previously that, in the presence of severe disc degeneration, the neural arch can resist up to 80% of the compressive force acting on the spine. We hypothesise that the inferior articular processes can then act as a “pivot” during backward and lateral bending movements.
Materials and Methods: Twenty-one motion segments (T8–9 to L4–5) were obtained from spines aged 48–90yrs. Specimens were loaded rapidly to simulate flexion, extension and lateral bending, while vertebral movements were tracked using an optical MacReflex system. The varying position of the centre of rotation (CoR) during these movements was calculated. Experiments were repeated after a treatment designed to simulate two effects of severe disc degeneration: creep loading to dehydrate the disc, and compressive overload to fracture a vertebral endplate and decompress the nucleus.
Results: In flexion, the CoR was usually located just below the inferior endplate of the disc, close to the antero-posterior midline, and in extension it moved an average 4.6 mm posteriorly. The additional “disc degeneration” treatment increased the variability of the CoR within and between specimens. It also moved the CoR an average 10.7mm posteriorly during extension movements (P< 0.001), so that in some specimens it was near the tip of the inferior articular processes.
Discussion: Severe disc decompression and narrowing increase translational (gliding) movements between adjacent vertebrae so that the effective CoR becomes more variable. During extension movements, the CoR can move so far posteriorly that the vertebrae can effectively “pivot” about the inferior articular processes.
Correspondence should be addressed to SBPR c/o Royal college of Surgeons, 35 - 43 Lincoln’s Inn Fields, London WC2A 3PN
