Stresses at the lumbosacral junction.

This report has presented a discussion of the mechanics of the spine in terms of an analogy with a curved, tapered beam. We have identified the existence of critical frequencies and locations in the spine and discussed the evidence from clinical experience with low back problems. We have also discussed the extrinsic mechanisms for protecting the spine and the lumbosacral junction. The question of the sensitivity of the lumbosacral junction to shear forces has been raised and experiments described that show that the shear forces that will injure the posterior elements of the fifth lumbar vertebra are low and occur commonly. The report also describes an instrument to measure the stiffness of articulations of the spine.