Fatigue fracture morphology in human lumbar motion segments.

The fatigue behavior of 17 fresh lumbar spine motion segments was examined during cyclic, axial, compressive loading. The loading frequency and magnitude were chosen to simulate vigorous physiological loading conditions. The average failure occurred after 318 such loading cycles. Two main types of failure occurred and both involved the endplate and the adjacent subchondral spongy bone of the vertebral body. A tendency was observed toward a relation between the degree of disk degeneration and fracture type. Location of the fatigue fractures corresponded to the location of earlier detected microcalluses of the spongy trabeculae in the vertebral body. The findings further demonstrated that, under repetitive physiological loading, the weakest part of the lumbar motion segment was the endplate and the adjacent spongy bone.