Primary instability of lumbar vertebrae as a common cause of low back pain.

1. Primary lumbar vertebral instability or "pseudo-spondylolisthesis" varies from about 3 millimetres to 1·7 centimetres, and is perhaps the commonest radiological sign associated with lumbo-sacral pain after the third decade of life. It was observed in 28·6 per cent of 500 consecutive cases of lumbo-sacral pain. The next commonest cause is gross disc degeneration, which is a late result of instability. 2. The secondary instability that may accompany a nuclear prolapse or osteoarthritis is excluded from this discussion. 3. This lumbar instability is an early sign of "incipient disc degeneration," occurring before narrowing of the disc space, sclerosis of the epiphysial rings, or osteophyte formation becomes evident. The instability in the lower lumbar region is caused by incomplete radial posterior tears, usually between the fourth and fifth lumbar vertebrae; and in the upper lumbar region from anterior concentric fissures or slits between some of the lamellae of the annulus fibrosus. 4. As shown radiologically, lumbar instability is commonest between L.4-5 and is rare between L.5 and sacrum because the facets between L.5-S.1 normally face forwards and backwards and thus resist anterior sliding. 5. The usual direction of antero-posterior sliding in the case of the upper four lumbar vertebrae is posterior—that is, the upper vertebra is displaced backwards on the one immediately below it during full extension in the erect position. The displacement tends to disappear on forced flexion, which may cause anterior displacement. On the other hand, the reverse displacement may exist between the fifth lumbar vertebra and the sacrum. 6. Operative treatment by bone grafting is a last resort in carefully selected individuals. After operation the patient rests in bed for three months without rigid splinting. Bone grafting is best for a localised lesion (affecting only one disc); it is generally not advisable if more than two discs are involved. 7. The results in thirty patients treated by spinal fusions showed that 70 per cent had no pain and resumed work, l3·3 per cent had improvement and resumed work, and l6·7 per cent were worse or no better.