The myth of lumbar instability: the importance of abnormal loading as a cause of low back pain

Spinal fusion became what has been termed the “gold standard” for the treatment of mechanical low back pain, yet there was no scientific basis for this. Operations of fusion for low back pain were initially done at the beginning of the last century for back pain thought to be related to congenital abnormalities or for past spinal infection. The recognition of the disc as a cause of sciatica, commonly associated with back pain, and the recognition that a degenerate disc led to abnormal movement suggested the concept that this abnormal movement was the cause of pain, and this abnormal movement came to be called “instability”. Much biomechanical expertise confirmed the fact that degenerate discs led to abnormal movement, there were many hypothesis as to why this caused pain. However clinical results of fusion for back pain were unpredictable. The failure of pedicle screws and cage fusion to improve the clinical results of fusion despite near 100% fusion success, and the introduction of “flexible stabilization” and artificial discs, which demonstrated that despite the often unpredictable movement permitted by of these devices, clinical success was similar to fusion, directed attention to the other role of the disc, that of load transfer, which these devices also affected. Abnormal load transfer was already known to be critical in other joints in the body and had led to the use of osteotomy to realign joints. The relevance of load transfer to the future design of spinal implants used in the treatment of low back pain is discussed, and some finite element studies are reported demonstrating the likely effect of abnormal loading beneath an incompletely incorporated plate of an artificial disc, perhaps explaining in part the somewhat disappointing clinical results to date of the implantation of artificial discs.