The dynamic response of L(2)/L(3) motion segment in cyclic axial compressive loading.

OBJECTIVE: The dynamic response and load sharing amongst passive elements of an L2-L3 motion segment during axial compressive cyclic loading was investigated. DESIGN: A validated viscoelastic nonlinear finite element model of L2-L3 was used for a detailed stress/strain analysis during axial cyclic loading. BACKGROUND: The repetitive loading of the spine has been implicated as a risk factor in developing low back disorders. However, the quantitative description of injury mechanisms and the internal load sharing have been lacking. METHODS: The applied cyclic axial compressive loading was controlled, peak to peak, from 600 to 1000 N at 0.5 Hz for 15 cycles. The stress/strain and strain energy density of various elements were quantified and the effects of cyclic loading on these parameters were investigated. RESULTS: The axial stiffness of the motion segment decreased, while intradiscal pressure (IDP) and the strain in anulus fibers of the outermost lamella increased. The axial stresses of outer lamellae in the anulus matrix reduced, in contrast to the increased strain at the endplate. CONCLUSIONS: The load sharing amongst the passive elements of the motion segment changed. The response of the motion segment to the same external axial load depends on the history of loading. The anulus fibers in the innermost layer were slack due to compression, hence not at risk of failure. The loss of disc height and increased disc bulge led to higher strain in anulus fibers of outermost layer. In future, more complex loading conditions with a longer duration should be considered.