Rate-related fatigue injury of vertebral disc under axial cyclic loading in a porcine body-disc-body unit.

OBJECTIVE: Cyclic loading tests were performed on fresh mature porcine lumbar spines to investigate the mechanical responses at different loading rates, morphological changes in the disc and to produce the clinically relevant injuries of porcine intervertebral disc. DESIGN: Two-segment units of fresh porcine spine with all the posterior elements removed were used for a disc stress analysis and morphological observation during axial cyclic loading. BACKGROUND: The repetitive loading of the spine has been implicated as a risk factor in developing low back disorders. However, few studies have discussed morphologic changes in vertebral disc due to fatigue stress and no study has investigated the relationship between disc strength and bone mineral density. METHODS: 21 body-disc-body specimens divided into three groups were subjected to cyclic loading at test speeds of 0.5, 5 and 20 mm min(-1). The correlation between residual stress of specimen and bone mineral density was investigated and the disc morphological changes were observed through the middle cross section of the intervertebral disc using a zoom stereo microscope. RESULTS: There was no visible crack observed following testing in the 0.5 mm min(-1) test group. However, one disc in the 5 mm min(-1) and two discs in the 20 mm min(-1) test group exhibited disc herniation. The residual stress was found to be positively and linearly correlated with bone mineral density and decreased as the loading rates increased. CONCLUSION: Faster loading rate generates greater stress decay, and disc herniation is more likely to occur under higher loading rate conditions. The most common site of disc herniation is the posterolateral area which is in agreement with that of the human spine.