A Finite Element Study On Dynamic Characteristics Of The Human Whole Lumbar Spine Under Vibration

In recent years,with the rapid development of economy and society,vehicles have become the main means of transportation for people to travel.Vehicles have brought great convenience to people's life,but they also give the people a more frequent exposure to whole body vibration(WBV).In view of this,the effect of WBV on human health has obtained more and more attention from the researchers.Epidemiological studies have suggested that long-term vibration exposure constitutes a serious threat to spine health and significantly contributes to disc degeneration and low back pain,which leads to work disability and reduced quality of life.Therefore,it is of great scientific significance to explore biomechanical characteristics and injury mechanism of the human spine under vibration.However,previous studies mainly focused on investigating biomechanical characteristics of the human spine under static loading conditions.Very few of studies have comprehensively quantitatively analyzed vibration characteristics of the whole lumbar spine in the presence of a physiologic compressive preload.In order to investigate the effect of WBV on dynamic characteristics of the human spine,and the effect of injury spine on its adjacent components under vibration,a detailed three-dimensional non-linear finite element(FE)model of the human whole lumbar spine(L1-S1)was developed in this paper.Some injury and degenerative conditions of the lumbar spine were also simulated.The physiologic compressive load on the whole lumbar spine was applied using the follower load technique.The dynamic response results of each spinal level to the vibration were computed using the developed FE model and compared.The contents of this paper are as follows:(1)The bony geometry of the human spine was obtained from CT scans of a volunteer without any spinal disease.The obtained geometrical data was saved as DICOM format and imported into the Mimics software to extract the image of L1-S1 motion segments and reconstruct its three-dimensional CAD model.Based on anatomical structure of the lumbar spine,the intervertebral discs and ligaments were added into the CAD model in FE software ABAQUS,and then to generate a complete FE model of the human whole lumbar spine.(2)Before performing FE analyses using the present model,the model should be validated in order to ensure accuracy of the computed results.The model validation process consisted of two parts:static and dynamic validations.For the static validation,the model predictions for various static loading(axial compression,flexion,extension and lateral bending)were consist with the experimental results in the literature.For the dynamic validation,response results of the model to free and forced vibration were also in accord with the corresponding published experimental results.These results indicate that the present FE model could be used for further analyses on lumbar biomechanical characteristics.(3)Free vibration(modal)analyses were conducted on the developed FE model of whole lumbar spine.The obtained dynamic response results in terms of resonant frequency and mode shape were used to study vibration trend of the lumbar spine and find the spinal components that suffer more harms from vibration,and thus to avoid from exposure to some potential dangerous vibration environments and prevent the spine vibration-related injuries.In addition,this study simulated some typical injury conditions of the spine.Modal studies were also conducted on these injury models and to analyze their dynamic characteristics.(4)Forced vibration analyses were conducted on the developed FE model of whole lumbar spine.Time-domain stress and strain responses of the spinal levels to axial cyclic vibration loading were analyzed to determine effect of the dynamic load on dynamic characteristics of the lumbar spine.As a comparison,the corresponding results for static loads were also computed.The results showed that compared the corresponding static load,dynamic load markedly increased stresses and strains within the discs.This implies that the dynamic load increases the risk of lumbar degeneration and injuries.In addition,this study investigated effects of the related parameters such as axial compressive preload and cyclic loading frequency on time-domain dynamic response of the lumbar spine.(5)On the basis of the healthy lumbar model,FE models of the lumbar spine in different grades of degeneration were developed by changing geometry and material properties of the L4-L5 level and validated.Then,forced vibration analyses were conducted on these degenerative models.Through comparison of the time-domain dynamic responses of stresses and strains within the discs between the healthy and degenerative models,the effect of single-level disc degeneration on dynamic characteristics of the whole lumbar spine was determined.(6)Sensitivity studies of material properties on lumbar spine responses under vibration were conducted using the developed FE model.The effect of variations in material properties of vertebra and intervertebral disc on dynamic characteristics of the whole lumbar spine was analyzed.