Influence of muscle soft tissue and lower limbs on the vibration behavior of the entire spine inside the seated human body: A finite element study.

The objective of the study is to investigate the vibration behavior of the entire spine inside the human body and the influence of muscle soft tissue and lower limbs on spinal response under vertical whole-body vibration. This study conducted modal and random response analyses to simulate the modal displacements and stress of all intervertebral discs in the vertical principal mode in the skeleton, upper, and whole body. Additionally, the acceleration response of intervertebral discs under vertical random excitation was investigated. The results revealed that removing muscle soft tissue and lower limbs significantly changed the resonant frequency, modal displacement, and stress. Particularly, there was a rapid increase in vertical displacement of the lumbar spine in the skeleton model. The reason for that was due to the lack of soft tissue to provide stability, leading to significant lumbar spine bending. Under random excitation, the fore-aft acceleration of intervertebral discs in the skeleton model was considerably larger than that in the whole body, especially in the lumbar spine where it can reach up to four times higher. Conversely, the vertical response of the intervertebral discs inside the human body model was 1.4-2.4 times larger than that of the skeleton model. Muscle soft tissue contributes to the strength of the spine, reducing fore-aft response. The muscle soft tissue in the gluteal region, connected below the spine, can lower the vertical natural frequency and attenuate spinal impact. Although the lower limbs enhance spinal stability, stimulation from the feet can superimpose vibrational responses in the spine.

[Effect of fiber volume and material property and nucleus pulposus area on intervertebral disc mechanical behavior].

The material properties and volume proportion of the fibers as well as the cross-sectional area proportion of nucleus pulposus vary greatly in different studies. The effect of these factors on the mechanical behavior of intervertebral discs (IVDs) are uncertain. The IVDs finite element models with different parameters were created to investigate the pressure, height, rotation, stress, and strain of the IVDs under loads: pure compression, rotation after compression or axial moment after compression. The results showed that the material properties of fibers had great impact on the mechanical behavior of IVDs, especially on the rotation angle. When the fiber volume ratio was small, its changes had a significant impact on the rotation angle of the IVDs. The area proportions of nucleus pulposus had relatively little effect on the mechanical behavior of IVDs. The IVDs rotation should be observed when validating the model. By adjusting the elastic modulus or volume ratio of fibers within a reasonable range, a model that could simulate the mechanical behavior of normal IVDs could be obtained. It was reasonable to make the area proportion of nucleus pulposus within 25%-50% for the IVDs finite element model. This study provides guidance and reference for finite element modeling of the IVDs and the investigation of the IVDs degeneration mechanism.

A finite element method study of the effect of vibration on the dynamic biomechanical response of the lumbar spine.

BACKGROUND: Studies focusing on lumbar spine biomechanics are very limited, and the mechanism of the effect of vibration on lumbar spine biodynamics is unclear. To provide guidance and reference for lumbar spine biodynamics research and vibration safety assessment, this study aims to investigate the effects of different vibrations on lumbar spine biodynamics. METHODS: A validated finite element model of the lumbosacral spine was utilized. The model incorporated a 40 kg mass on the upper side and a 400 N follower preload. As a comparison, another model without a coupled mass was also employed. A sinusoidal acceleration with an amplitude of 1 m/s2 and a frequency of 5 Hz was applied to the upper and lower sides of the model respectively. FINDINGS: When the coupled mass point is not introduced: in the case of upper-side excitation, the lumbar spine shows a significantly larger response in the x-direction than in the z-direction, while in the case of lower-side excitation, the lumbar spine experiences rigid body displacement in the z-direction without any movement, deformation, rotation, or stress changes in the x-direction. When the coupled mass point is introduced: both upper and lower-side excitations result in significant differences in z-directional displacement, with relatively small differences in vertebral rotation angle, disc deformation, and stress. Under upper excitation, low-frequency oscillations occur in the x-direction. In both types of excitations, the anterior-posterior deformation of the L2-L3 and L4-L5 intervertebral discs is greater than the vertical deformation. The peak (maximum) disc stress exceeds the average stress and stress amplitude across the entire disc. Regardless of the excitation type, the stress distribution within the disc at the moment of peak displacement remains nearly identical, with the maximum stress consistently localized on the anterior side of the L4-L5 disc. INTERPRETATION: Accurately simulating lumbar spine biodynamics requires the inclusion of the upper body mass in the lumbosacral spine model. The physiological curvature of the lumbar spine could escalate the risk of lumbar spine vibration injuries. It is more instructive to apply local high stress in the disc as a lumbar spine vibration safety evaluation parameter.