A more realistic disc herniation model incorporating compression, flexion and facet-constrained shear: a mechanical and microstructural analysis. Part I: Low rate loading.

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To date, the mechanisms of disc failure have been explored at a microstructural level in relatively simple postures. However, in vivo the disc is known to be subjected to complex loading in compression, bending and shear, and the influence of these factors on the mechanisms of disc failure is yet to be described at a microstructural level. The purpose of this study was to provide a microstructural analysis of the mechanisms of failure in healthy discs subjected to compression while held in a complex posture incorporating physiological amounts of flexion and facet-constrained shear. 30 motion segments from 10 healthy mature ovine lumbar spines were compressed in a complex posture intended to simulate the situation arising when bending and twisting while lifting a heavy object, and at a displacement rate of 40mm/min. Nine of the 30 samples reached the predetermined displacement prior to a reduction in load and were classified as early-stage failures, providing insight into initial areas of disc disruption. Both groups of damaged discs were then analysed microstructurally using light microscopy. Complex postures significantly reduced the load required to cause disc failure than earlier described for flexed postures [8.42kN (STD 1.22kN) compared to 9.69kN (STD 2.56kN)] and resulted in a very different failure morphology to that observed in either simple flexion or direct compression, involving infiltration of nucleus material in a circuitous path to the annular periphery. The complex posture as used in this study significantly reduced the load required to cause disc failure, providing further evidence that asymmetric postures while lifting should be avoided if possible.