Vibration characteristics of the human spine under axial cyclic loads: Effect of frequency and damping

Study Design. A nonlinear finite element model of lumbar spine segment L3 - L5 was developed. The effects of upper body mass, nucleus injury, damping, and different vibration frequency loads were analyzed for the whole body vibration. Objectives. To analyze the influence of whole body vibration on facets of lumbar spine and to analyze the influence of nucleus injury, upper body mass, and damping on the dynamic characteristics of lumbar spine. Summary of Background Data. Many studies have investigated whole body vibration for lumbar spine. However, very few investigations analyzed the influence of whole body vibration on facets and vibration characteristics of the injured spine. Methods. The nonlinear finite element model of the L3 - L5 segment was constructed based on the embalmed vertebra geometry and validated. Besides static and modal analyses, transient dynamic analyses were also conducted on the model with an upper body mass under damping and different frequency cyclic loads. Results. In the period of human spine vibration, the vibration effects of different regions of the lumbar spine are not the same. Anterior regions of the L3 - L5 segment show small vibration amplitudes, but posterior regions show large amplitudes. The vibration amplitude of facet contact force is more than 2.0-fold as large as that of displacement and stress on vertebrae or discs. To decrease the weight of the upper body will increase the resonant frequency. To remove the nucleus will decrease the resonant frequencies. The vibration displacement, stress, and facet contact force will reduce generally by 50% using damping ratio 0.08. Conclusions. The posterior regions of intervertebral discs of the lumbar spine are easy to injure during long-term whole body vibration compared to anterior regions. The vibration of human spine is more dangerous to facets, especially during whole body vibration approximating a sympathetic vibration, which may lead to abnormal remodeling and disorder of the lumbar spine.