Fatigue response of lumbar intervertebral joints under axial cyclic loading.

A low-cycle fatigue of 11 lumbar intervertebral joints under axial compression is reported. The magnitude of the maximum compressive load ranged from 37 to 80% of the failure load. The maximum deformation, as a function of the number of cycles, showed two distinct results: one group showed a gradual, stable increase, and the other an abrupt, unstable increase. The before- and after-test radiographs showed a one-to-one correspondence between unstable specimens and generalized bony failure. The radiographs of 5-mm thick transverse endplate slices show crack propagation from the periphery of the subchondral bone inward. Removal of the organic matrix from the cracked specimens produced its physical disintegration into small particles, while normal controls and stable specimens retained their size and shape.