Development And Application For A Ligamentous Cranial Vertebra-cervical Spine Finite Element Model With Active Muscles

Part 1 The establishment and analysis of a non-linear finite element model of the ligamentous cranial vertebra-cervical spineObjectiveThis study aimed to construct a non-linear ligamentous cranial vertebra-cervical spine of finite element model(FEM),which accurately expressed the mechanical response of the cervical spine,including the lower cervical spine motion unit and the occipital-atlas-axis motion unit,as the basis for the skeletal muscle modeling in head-neck motion.MethodsThe image data of a 23-year-old healthy adult male volunteer was used as the basis for model construction.Mimics 19.0 and Geomagic Studio 2015 were used to extract geometric entities of the cranial vertebra-cervical spine.The model components include bony tissue,ligament,articular cartilage,intervertebral disc,and spinal cord area.These above components were imported into the Solidworks 2017 for assemble and model interference inspection.Then,the assembly model was imported into Hypermesh 2019 to generate mesh and assign materials:the ligaments were set as the nonlinear orthotropic material;the nucleus pulposus was set as elastic fluid material;the annulus fibrosus was set as hyperelastic material with fiber layers of orthotropic material;the cortical bone and cancellous bone were set as isotropic elastoplastic material;the gray matter and white matter in cervical spinal cord were set as material with both viscoelastic and hyperelastic material.Then,the model was set up the pre-processing work such as property,contact relations and boundary conditions.The LSDYNA R11.0 explicit solver was chosen to verify the validity of the model.The verification process was divided into construct verification and component verification,including quasistatic and dynamic processes.Construct verifications were included:? Compression test of cervical spine functional unit(FSU);? Tensile test of each ligament of upper and lower cervical spine(UCS and LCS);?Quasi-static 6 degrees of freedom torque test in each cervical spine functional unit low-speed impact test of cervical spinal cord.?dynamic impact of cervical spinal cord.Component verifications were included:? Anatomical geometry measurement;?C2-T1 quasi-static flexion and extension torque test.The Frankfurt plane of the head and neck complex was aligned with the horizontal plane as the standard plane of the study.For all simulation processes,the gravity load was included.ResultsThe established three-dimensional non-linear FEM with cranial vertebra-cervical spine was excellent geometric similarity.The force-displacement curve included the FSU compression,extension of UCS and LCS ligament,and impact of the cervical spinal cord,which were similar to the results of cadaveric experiments.Under the torsion verification of the cervical spine with 6 degrees of motion,the motion range of the simulation model was generally suitable for in vitro experiments.ConclusionA non-linear FEM of the ligamentous cranial vertebra-cervical spine has established,which satisfies the geometric similarity and mechanical similarity in finite element analysis.It has the characteristics of expressing the mechanical response of the head-neck,and it can be used for the biomechanical analysis of the cervical spine.Part 2 Structural construction of a muscle model of the cranial vertebra-cervical spine ObjectiveThis study aimed to construct the main cervical spine muscles,which affiliated to the original cranial vertebra-cervical spine model.After verification,this model was used to investigate the influence of passive muscles(PM)' characteristics on the range of motion of the head-neck and the mechanics responses of adjacent soft tissues.MethodsThe Mimics 19.0,Geomagic Studio 2015 software were used to extract the hyoid bone,clavicle,scapula,and thorax as the starting and ending points of the muscles.Then,these above constructs were imported into Solidworks 2017 for assemble,and Hypermesh 2019 was used to generate meshes and assign materials,properties,and contact relationships.1-D beam units were used to build the muscles in the original cranial vertebra-cervical spine model.Then,the equivalent physiological cross-sectional area(PCSA),and the starting and ending points of the muscles were referred to classic anatomical studies.A total of 32 groups of main head-neck muscles were constructed,and ventral cervical muscles were defined as flexors and dorsal cervical muscles were defined as extensors.Contractile force-length curve Fl(l),contractile force-speed curve Fv(v),maximum stress ?max and maximum isometric contraction force Fmax were set.At the same time,passive elastic relationship shape factor CPE and PEmax were set to complete the Hill muscle unit.Then,the LS-DYNA R11.0 explicit solver was used.The 4g rear impact of postmortem human subjects(PMHS)was selected for present model's verification,and gravity load was included.The total simulation time was 250ms.Head rotation angle-time curve of simulated model was compared to PMHS data.After verification,the original model without muscle unit was simulated with the same boundary conditions,and compared with present PM-FEM.The differences of head angletime curve,stress and strain in adjacent tissues between two models were observed.The mechanical effects of the PMs' characteristics on the Head angle-time curve,the peak Von Mises stress of cancellous bone,the peak shear stress of intervertebral discs,the peak strain and Von Mises stress of cervical spinal cord,and the peak strain of UCS and LCS ligaments were observed.ResultsPresent PM-FEM with Hill muscle unit was well adapted to dynamic verification under high-speed motion and reflected the structural stability.The curve of head angle-time between present model and PMHS experimental was relatively similar,and the peak value of the mechanical response was within the data corridor of PMHS.Compared with PM-FEM,the head angle-time curve of original model was basically similar,and the time of peak value between two models was parallel,too.However,the peak angle of the PM-FEM was about 7°less than the original FEM,and PM-FEM possessed 6.48%restriction in range of motion(ROM).The original model exceeded the data range of the PMHS experiment.At the same time,the retraction amplitude of the head rotation angle of the original model was less than that of the present model?The former retracted 4.04°,and the latter retracted 8.41°.Color nephograms of cancellous bones,intervertebral discs,and cervical spinal cord between two models showed that the stress concentration sites of the two models were roughly parallel,but the PM-FEM stress was more concentrated.Compared with the original FEM,PM-EFM possessed 1.14-2.26 times more peak Von Mises stress in cancellous bone.In the intervertebral discs,PM-FEM produced 1.05-1.70 times more peak shear stress.In the cervical spinal cord,PM-FEM possessed 1.03-1.22 times more peak strain and 1.23-2.55 times more peak Von Mises stress.Thus,PM-FEM was subjected to greater stress than the original FEM,while more peak strain of UCS and LCS ligaments was suffered,too.ConclusionIn this study,the Hill muscle unit is constructed based on the original finite element model of cranial vertebra-cervical spine,and it has been verified under the condition of rear impact.In addition,the passive nature of the present muscle model possesses a certain effect on the ROM in head-neck area and the mechanical response in adjacent soft tissues:?Without the activation of the neuroregulatory,the material properties(passive behavior)of PMs added by FEM can restrict the joints movement.?Muscle materials can produce indirect loads on adjacent structures such as bones and intervertebral discs.When subjected to loads,tensioned muscles will increase the effective stiffness of adjacent structures such as vertebral bodies and intervertebral discs,resulting in a greater concentration of stress and strain in the tissue.Part 3 Active muscle dynamic stability based on the feedback of proprioceptors and vestibular system:application in head and neck hyperextension injuryObjectiveIn order to analyze the mechanical response of the biological tissue under the human biomechanical model,this study aims to construct a dynamic stabilization system of active muscle,which is based on the finite element model with passive muscles.MethodsAn accelerative curve of rear impact from a volunteer experiment was calculated by an OpenSim skeletal-muscle model with the muscle proprioceptor and vestibular system frame.Then,the active muscle activation function A(t)of the ventral flexors calculated by the self-balancing framework were imported into LS-DYNA,and correlated with each muscle to construct FCE.There were 12 groups of flexor muscles with different degrees A(t),which included longus capitis,longus colli,sternocleidomastoid-mast,sternocleidomastoid-clav,scalenus anterior,scalenus medius,scalenus posterior,geniohyoid,mandiohyoid,omohyoid,sternohyoid,and stylohyoid muscles.Then,the data corridor of volunteer experiment was selected to verify AM-FEM,and the total simulation time was 225ms.The head angle-time curve obtained from the simulation model was compared with the data corridor of volunteer experiment.The PM-FEM was simulated with the same boundary conditions.Then,AMFEM and PM-FEM were compared to some biomechanical indexs under the dynamic stabilization system of active muscle:? Head angle-time curve and working condition between two models;? The peak Von Mises stress and color nephograms,peak shear stress and peak pressure stress of each cancellous bone;?The peak Von Mises stress and color nephograms,peak shear stress and peak pressure stress of each intervertebral discs;?The peak Von Mises stress and color nephograms,peak shear stress and peak pressure stress of cervical spinal cord;?Peak strain of UCS and LCS ligaments.ResultsThe trend of Head angle-time curve of AM-FEM was similar to the data corridor of volunteer experiment,and peak value of the curve was within the experimental corridor.The peak angle of the curve of AM-FEM was-39.64° in the loading condition,which decreased 25.11°compared with PM-FEM,and the ROM was limited by 38.78%.Since PM-FEM did not produce muscle activation,its curves were generally outside the experimental data.The working condition was showed that a slightly forward buckling of the neck was appeared in AM-FEM at 50-100ms.As the acceleration increased,AM-FEM retracted and gradually extended backward after 100ms,and the angle reached maximum at 225ms.In the comparison of the mechanical response in each cancellous bone,it was found that the distribution area of AM-FEM's color nephogram was significantly smaller than that of PM-FEM.In addition,the peak Von Mises stress,the peak shear stress,and the peak compressive stress of AMFEM were 1.43 to 3.04 times,1.25 to 3.00 times,and 2.83 times less than that of PM-FEM.In the comparison of the mechanical response in each intervertebral disc,AM-FEM in the C2/3,C3/4 disc suffered less peak Von mises peak stress,peak shear stress and pressure stress,but these results were opposite in the C4/5,C5/6,and C6/7 disc.Although color nephograms showed that stress area of PM-FEM was more extensive,the high peak stress was displayed in the posterior area of some intervertebral discs.In C2/3 and C3/4 intervertebral discs,AMFEM was subjected to 1.71 and 1.21 times less peak Von Mises stress,1.21 and 1.74 times less peak shear stress,and 1.16 and 1.14 times less peak compressive stress than PM-FEM,respectively.In C4/5,C5/6 and C6/7 intervertebral discs,AM-FEM was subjected to 1.38,1.12 and 1.18 times of peak Von Mises stress,1.37,1.32 and 1.18 times of peak shear stress,and 1.50,1.44 and 2.22 times of peak compressive stress,respectively.In the comparison of white matter and gray matter between two models,the peak strain of AM-FEM was 2.34 and 1.37 times less than that of PM-FEM,and the peak shear strain was 2.33 and 1.34 times less than that of PM-FEM.Color nephograms also indicated that the strain distribution area of AM-FEM is significantly smaller than that of PM-FEM.Compared with PM-FEM,AM-FEM was subjected to 4.48 times and 21.39 times peak Von Mises stress,3.61 times and 20.29 times peak shear stress,and 3.15 times and 3.76 times peak compressive stress,respectively.In the comparison of the mechanical responses of ligaments,AM-FEM was subjected to smaller peak strain of ligaments than PM-FEM,which produced 21.82%to 61.61%restraction of peak strain in UCS ligaments and 0.84%to 50.35%restraction in LCS ligaments.ConclusionThe dynamic stabilization of AMs limits excessive bone-joint ROM.In addition,AMFEM reduces stress,strain in adjacent tissues in the cranial vertebra-cervical spine,and AMFEM decreases load distribution under external loading.The dynamic stabilization of AMs can also reduce the damage tolerance of biological tissue,and possesses the protective effect on the structure of head-neck area.However,AMs activation may increase compression fore to intervertebral discs and produce an extra load on ligaments in LCS.