BIOMECHANICS OF LUMBAR INSTABILITY

Several authors have tried to define segmental lumbar instability. The ir definitions: increased antero-posterior translation, pathologic cou pled motion, increased neutral zone, pathologic instantaneuous center of rotation describe some mechanic findings occuring in the aging spin e. However, there is no evidence that they help to differentiate the p athologic entity of segmental lumbar instability from the normal aging process. Dynamic explanation models are promising, but at the moment they cannot be used clinically for diagnosis of instability as well. T he most important structure to maintain lumbar stability is the interv ertebral disc. In the third and fourth decade, more than 50 percent of specimen show peripheral tears of the anulus. It was shown in animal experiments that these tears develop to radial tears, which are accomp anied by nuclear volume loss and decreased height. The facets degenera te one or two decades later. Corresponding with the loss of discal fun ction, they increasingly contribute to spinal stability. In conclusion , the concept of lumbar segmental instability is not very helpful in c linical practise. It is recommended to base the decision of lumbar fus ion on a painful degenerated disc, and additional findings promising a good result.