| The upper cervical spine,including the occipitocervical part,is an important structure connecting the head and the lower cervical spine.Because there is no strong restraint of the lower cervical intervertebral disc structure,the upper cervical spine can be stabilized by ligaments and muscles.Therefore,the upper cervical spine has a great degree of freedom and can achieve flexion,extension,lateralbending and axial rotation.The literature shows that the upper cervical spine takes up 50% of the total cervical motion.With the development of society,the traffic accident rate is increasing year by year.Long-term overwork,bowing head to play with mobile phones and so on have caused serious injury to cervical spine,resulting in cervical fracture,ligament calcification,disc herniation,cervical curvature reduction and other cervical diseases.Other pathogenic vertebral diseases,such as rheumatoid diseases and tumors,will also plague people’s lives.Upper cervical spine injury and disease accounted for 50% of the total cervical spondylosis.Because of the importance of connecting the head,it has become the focus of spinal surgeryresearch.In this paper,Mimics software is used to reconstruct the upper cervical CT files.Several files are imported into ANSYS-ICEM.Hypermesh software is used to mesh and complete the finite element model of the upper cervical spine.ABAQUS was used for finite element analysis.The boundary condition was set to restrict six degrees of freedom on the lower surface of C3 vertebral body.The simulated head gravity was applied vertically down to 40 N on the occipital bone and 1.5N m moment was applied at the same position to simulate the effect of cervical muscle force on pulling the head.The basic movement of cervical spine in four directions,namely,forward bending,backward bending and rotation,was produced.The results show that the mobility and stress distribution of each segment of cervical spine can be obtained.The validity of the model is validated by comparing the results of in vitro experiments and finite element models published in the literature.On the basis of this model,the instability model caused by Hangman fracture,single screw titanium plate fixation + cage bone graft fusion model(Singlescrew Plate + Cage,SPlate + Cage)and double screw titanium plate fixation + cage bone graft fusion model(Doublescrews Plate + Cage,DPlate + Cage)were established.In order to determine the advantages and disadvantages of the two methods of internal fixation,the differences of mobility,stress distribution of vertebral body and stress distribution of internal fixator were studied under the same loading mode and boundary conditions.The range of motion of C0-C1,C1-C2 and C2-C3 segments were 11.2 degree,9.1 degree and 3.1 degree under flexion condition,9.2 degree,14.2 degree and 3.5degree under extension condition,5.4 degree,5.0 degree and 4.1 degree under lateral flexion condition,1.8 degree,24.5 degree and 2.5 degree under rotation condition,respectively,which were within the scope of literature data.Both groups of internal fixation models can significantly reduce C2-C3 segment mobility and achieve good stability.Compared with the ROM of instability model,the ROM of SPlate+Cage model decreased by 94.9%,96.0%,92.9%,91.6% and the ROM of DPlate+Cage model decreased by 95.9%,97.3%,97.3% and 97.3% respectively.Both kinds of internal fixation have better stability in flexion and extension directions,DPlate+Cage has better stability in lateral flexion and rotation directions.The peak stress of SPlate+Cage model was greater than that of DPlate+Cage model under four working conditions.Under lateral bending condition,the stress of SPlate+Cage model was concentrated at the junction of screw and cortical bone.If the patient had severe osteoporosis,there was a risk of screw falling off.The stress of DPlate+Cage model is concentrated at the joint of screw and titanium plate.The structure of titanium plate and screw is stable,and the internal fixation of DPlate+Cage is safer.SPlate+Cage internal fixation simplifies the design and can shorten the operation time and patient wounds.Based on CT scanning documents and Mimics software,the geometric model with high similarity was obtained.The geometric model was meshed by ANSYS-ICEM and Hypermesh.A finite element model of upper cervical vertebra similar to the real human upper cervical vertebra was established.On this basis,Hangman fracturemodel and two kinds of internal fixation models were established.From the degree of motion,stress distribution and the complexity of operation,the model was established.The advantages and disadvantages of the two kinds of internal fixation were analyzed in several aspects,which provided some basis for clinical practice.Based on the internal fixation scheme proposed in this study for Hangman fractures,the dynamic response analysis of the finite element model of the upper cervical spine under the impact was developed to simulate the structural response of the vertebral body and the internal fixation part after the internal fixation of fracture patients under the accidental dynamic collision.Six degrees of freedom on the surface of the model under the C3 were completely constrained,an initial speed of 10 m/s was added in the center of the head geometry level at a total of five in the direction to simulate a head and neck in generally close to the car speed range of real postoperative cervical activity and accidental collision,etc.Simulation results show that internal fixation device and vertebral body stress distribution are showed similar features,i.e.,when the initial velocity is along the vertical downward direction the biggest peak stress occurred,and relatively weakened cross section after internal fixation can lead to stress concentration in the process of load transfer,the result can provide certain theoretical basis for patients with postoperative care. |
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