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 lo ading rates, morphological changes in the disc and to produce the clin ically relevant injuries of porcine intervertebral disc. Design. Two-s egment units of fresh porcine spine with all the posterior elements re moved were used for a disc stress analysis and morphological observati on 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 corr elation between residual stress of specimen and bone mineral density w as investigated and the disc morphological changes were observed throu gh the middle cross section of the intervertebral disc using a zoom st ereo microscope. Results. There was no visible crack observed followin g 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 di sc herniation. The residual stress was found to be positively and line arly correlated with bone mineral density and decreased as the loading rates increased. Conclusion. Faster loading rate generates greater st ress decay, and disc herniation is more likely to occur under higher l oading rate conditions. The most common site of disc herniation is the posterolateral area which is in agreement with that of the human spin e. Relevance Bone mineral density may be useful as an indirect indicat or to predict disc strength and stress decay by the good correlation b etween bone mineral density and residual stress. The disc injuries (te ar and herniation) under cyclic loading imply that fatigue failure and degeneration or instability are strongly linked. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.