The mechanical response of the neural arch of the lumbosacral vertebra and its clinical significance.

To investigate the effect of mechanical stresses in the aetiology of spondylolisthesis a study of the behaviour of the lumbar spine under load has been carried out. The loading tests on the lumbo-sacral segment, both with and without the dorsal elements present, showed that generally the disc only transmitted axial loads whereas transverse loads are resisted by the dorsal processes. Only after simulating a spina bifida, by splitting the lamina, was it possible to initiate a fracture in the isthmus by means of force applied in the transverse direction. Measurement of the stress behaviour in the neural arch shows that with transverse loading alone the pedicle can be just as heavily stressed as the pars interarticularis. Strain gauge measurements were performed on a macerated fifth lumbar vertebra to determine the manner in which loading forces must act to provide the greatest effect upon the region of the pars interarticularis. This was found to occur with vertical loading of the hyperextended spine. The vulnerability of the vertebra under these conditions was confirmed with further tests upon fresh cadaver specimens when it was found that the pars interarticularis of the fifth lumbar vertebra, when impinged upon by the caudal joint edges of the fourth lumbar vertebra, could be fractured with a vertically applied load of about 200 kp.