A Dynamic Finite Element Study On The Mechanism Of Cervical Spinal Cord Compression Caused By Degenerative Factors

As one of the most severe human injuries,spinal cord injuries are often associated with high mortality,disability,high treatment costs,and long-term rehabilitation,bring a heavy burden to both patient and society.Previous studies have indicated that spinal cord injuries mainly affect young adult males and the elderly population.The proportion of non-traumatic spinal cord injuries has begun to rise with the increasing aging population,with degenerative factors playing a increasing important role.As a result the effect of degenerative diseases on spinal cord injuries is attracting increasing attention.However,studies focused on this topic is pretty rare,resulting in a rough understanding on the injury mechanisms and prevention.The complexity of the cervical spine structure brings a lot of difficulties to the observation or measurement of injuries inside the spinal cord in cadaveric studies,and animal models alone with artificial materials are also ineffective in simulating the motion of the human cervical spinal cord combined with degenerative factors.As a result,there are no effective models available to analyze the biomechanical mechanisms involved in the dynamic activity of the cervical spine with degenerative factors,causing a bottleneck in the prevention,treatment,and research of related diseases.Spinal cord injury is generally caused by the deformation of the spinal structures around the cord.Advances in mechanics,mathematics and computational science have led to the emergence of computational simulation technology represented by finite elements,which can overcome various limitations of traditional experimental research and provide insight into the structure.The good scenario adaptability,research economy,and repeatability of this technique lights a new path for the biomechanical research.Applying the finite element method to the study of the influence of degenerative factors on the spinal cord can make up for the lack of experimental model in the mechanism research.Further understanding about the influence of degenerative factors on the internal stress of the spinal cord during cervical spine activities can be helpful for the diagnosis,treatment and prevention of related diseases.Therefore,this study aims to develop and validate a fluid-structure interaction cervical spine dynamic finite element model with spinal cord.Then simulate the effects of degenerative factors on the spinal cord in the presence of flexion and extension movements of the cervical spine and preliminarily analyze the effects of flexion and extension movements on the cervical spinal cord with the presence of degenerative factors.The effects of different degenerative factors on the spinal cord during cervical flexion and extension activities was also analyzed to explore the biomechanical mechanisms of the spinal cord compression by various degenerative factors during cervical flexion and extension activities.Methods and Results:1.This study firstly obtained scans of the cervical spine and spinal cord of young male volunteers without foreknown cervical degenerative disease by computed tomography and magnetic resonance imaging.The data were then imported into MIMICS for three-dimensional reconstruction,and the geometry was obtained and then trimmed and imported into Hyperworks for meshing.The vertebral body was modelled as a rigid body using triangular patches,the intervertebral disc,white matter,grey matter and cerebrospinal fluid were modeled using hexahedral elements,the ligamentum flavum,pial mater and dura mater were modeled using quadrilateral shell elements,the anterior longitudinal ligament,posterior longitudinal ligament,capsular ligament,spinous ligaments,intertransverse ligaments,denticulate ligaments and nerve roots were modeled using connector elements,and the dura mater surrounding the nerve roots was modeled using truss elements.The results indicate that the elements quality is good,the convergence test shows that the mesh density meets the requirement of this study,and the appearance of the model is similar to the geometry of the anatomy structures with good preservation of details.2.The model was assembled by selecting the appropriate constitutive model,material parameters and contact algorithm,then configuring the corresponding constraints and boundary conditions,and the model was configured according to the settings of the experimental study and simulated in ABAQUS.By comparing the results of the simulation with the data obtained from the experimental study,the rationality of the current model and the finite element method is verified.The results indicate that the model and the finite element method used in this study are in good agreement with the data from the experimental study and can meet the requirement for the investigation of the biomechanical mechanisms of degenerative factors compressing the spinal cord under flexion and extension activities of the cervical spine.3.Based on the validated model and finite element method,different degenerative factors(ossification of the posterior longitudinal ligament and ossification of the ligamentum flavum)were loaded on different segments of the cervical spine(C3,C4,C5 and C6)and the flexion and extension movements of the cervical spine were dynamically simulated in ABAQUS.The results suggest that flexion and extension of the cervical spine can intensify spinal cord compression,and that the stress and strain in the spinal cord increase with flexion or extension.The presence of degenerative factors can further increase this effect and lead to abnormal stress concentrations.4.By loading different types(two types of disc herniation,two herniation locations,two types of ossification of the ligamentum flavum)with different degrees(four degrees each and two degrees for the combined type)of degenerative factors and performing finite element simulations in the ABAQUS solver to compare and analyse the deformation as well as stress and strain magnitude and distribution inside the spinal cord during cervical flexion and extension activities.The results show that the spinal cord can move into the spinal canal space to avoid compression caused by lateral degenerative factors,while hypermobility can counteract this effect.Hyperextension or hyperflexion activities can lead to a sharp increase in stress in the spinal cord.Compression from both anterior and posterior sides can lead to stress concentrations inside the grey matter.5.In this study,a co-simulation technique was used to address the fluid-structure interation problem between the cerebrospinal fluid and the spinal cord.A dynamic finite element model of the cervical spine incorporating the spinal cord was developed for relevant simulations to investigate the effects of cervical flexion and extension alone with degenerative factors on the spinal cord.Promising a good application prospect in future fluid-structure interaction spinal cord injury simulation research.Conclusions:1.A fluid-structure interaction cervical spine finite element model incorporating the spinal cord has been successfully developed,the rationality and reliability of the developed model and the chosen finite element method was validated.This could make up for the lack of research models regarding the effects of degenerative factors on the spinal cord during cervical motion.2.Flexion and extension movements of the cervical spine can increase the stress and strain in the spinal cord and the presence of degenerative factors can lead to abnormal stress concentrations and possible impairment of motor function.Degenerative factors that do not cause spinal cord compression in neutral position may still cause spinal cord compression in flexion and extension,which may result in repeated spinal cord contusions that are hard to be detected by clinical imaging.3.There is always an interval during which the stress and strain was relatively low in the spinal cord when the cervical spine is doing flexion compared to extension,so appropriate forward flexion may be a good risk-averse position.4.A sharp rise in von-Mises stress inside the cord can be observed during hyperextension or hyperflexion compared to normal extension and flexion in combination with degenerative disease,suggesting that unexpected cervical motion can result in spinal cord injury.the spinal cord can evade from protrusions to reduce the maximum stress and strain inside the spinal cord,but this effect disappears during hyperactivity.5.Compression from both sides is essential to cause stress concentrations inside the grey matter,suggesting that the ligamentum flavum plays an important role in the development of central canal syndrome and that simultaneous anterior and posterior compression of the spinal cord should be avoided.6.Co-simulation technique has a good application prospect in the future research involving fluid-structure interaction simulation in spinal cord injury.