Material property sensitivity analysis on resonant frequency characteristics of the human spine.

The aim of this study is to investigate the effect of material property changes in the spinal components on the resonant frequency characteristics of the human spine. Several investigations have reported the material property sensitivity of human spine under static loading conditions, but less research has been devoted to the material property sensitivity of spinal biomechanical characteristics under a vibration environment. A detailed three-dimensional finite element model of the human spine, T12-pelvis, was built and used to predict the influence of material property variation on the resonant frequencies of the human spine. The simulation results reveal that material properties of spinal components have obvious influences on the dynamic characteristics of the spine. The annulus ground substance is the dominant component affecting the vertical resonant frequencies of the spine. The percentage change of the resonant frequency relative to the basic condition was more than 20% if Young's modulus of disc annulus is less than 1.5 MPa. The vertical resonant frequency may also decrease if Poisson's ratio of nucleus pulposus of intervertebral disc decreases.

Influence of anteroposterior shifting of trunk mass centroid on vibrational configuration of human spine.

This study attempts to determine the influence of anteroposterior (A-P) shifting of trunk mass from the upright sedentary posture on dynamic characteristics of the human lumbar spine. A three-dimensional finite element (FE) model comprising of the T12-Pelvis spine unit was used to mimic the human spine system. It is not clear how the A-P shifting of the upper part of human upper body affect on vibrational modality of the human lumbar spine under whole body vibration. Five trunk mass point locations were assumed by 2.0cm anterior, 1.0cm anterior, 1.0cm posterior and 2.0cm posterior to the upright sedentary posture including no shifting posture. FE modal analysis was used to extract the resonant frequencies and vibration modes of the human spine. The analytical results indicate that trunk mass centroid shifting onwards or rearwards may result in a reduction of vertical resonant frequency of the human spine. The human spine has the highest vertical resonant frequency at the normal upright sedentary posture with the trunk mass locating around 1.0cm anterior to the L3-L4 vertebral centroid. Larger A-P deformations and rotational deformations were also found at the spine motion segments L3-L4 and L4-L5, which imply higher compressive stress and shear stress at the disc annulus of those spinal motion segments. The findings in this study may explain why long-term whole body vibration might induce the degeneration of human spine at the relevant spinal motion segments.