The spine as an arch. A new mathematical model.

A new model is presented for the static behavior of the human spine that considers it to work as an arch rather than the traditional view of a cantilever. This theory is based on limit criteria, derived from plasticity theory, which determine bounds within which the structure is mechanically stable and thereby enables the prediction of failure when these criteria are not satisfied. It is shown that theorems developed for the plastic analysis of masonry arches can be simply adapted for the spine. An analysis is performed of three postures and associated loads described in the literature. The forces and intradiscal pressures are calculated and shown to be in good agreement with published measurements. The results show that compressive stresses in the spine are not as high as was previously calculated and that the curvature of the spine is necessary for its load-bearing function. Preservation of the lumbar lordosis, in conjunction with intra-abdominal pressure, strengthens the spine and is crucial to protect the spine from injury when lifting heavy loads.