| Objective:Cervical interbody fusion cage(cage)is a key device in cervical surgery,it’s function is to replace the degenerative intervertebral disc and stretch the vertebral body,effectively maintain the normal intervertebral space.However,due to the unreasonable stress distribution,it is easy to have the complication of cage subsidence after operation.In this paper,a customized cage was designed and the model of anterior cervical nucleus pulposus removal and bone graft fusion operation was established by simulation using three-dimensional finite element method,and the stress distribution of Von-Mises on cage,endplate and adjacent intervertebral disc after implantation of commercial cage and 3D printing customized cage was compared and analyzed,so as to evaluate the biomechanical properties of 3D printing cervical customized cage.The purpose of this study is to provide a theoretical basis for how to further prevent cage subsidence after cervical surgery.Methods:First of all,the heterogeneity finite element model of cervical vertebra is established.The three-dimensional CT data of the cervical vertebra of a 32-year-old normal female volunteer were collected and imported into the finite element analysis software Mimics,3-Matic and Magics.The reverse reconstruction,smoothing and denoising were processed,and then the data were imported into Hypermesh in STL format for mesh repair.According to the gray value of the CT data,each vertebral body was divided into10 kinds of density according to the gradient.After a series of operations,each vertebral body was divided into 10 kinds of density according to the grayscale value of the CT data.A three-dimensional heterogeneous finite element model of cervical vertebrae(C3～ C7)was established.After that,the ligament and intervertebral structure are established,the material attribute parameters are given,and the contact with the vertebral body is set.After the establishment of the heterogeneous finite element model of cervical vertebra,the preload of 73.6Nm and the motion torque of 1.0Nm were given to simulate the physiological working conditions of cervical vertebra.The range of motion(ROM)of each cervical segment in the motion state of flexion-extension and left-right bending under simulated load were recorded and compared with the data in the literature.T-test was performed in the two groups of data by using SPSS Statistics26.0 to verify the effectiveness of the model.After the model is built as a standby,the STL file of commercial cage is established and a customized cage is designed.Then select the model C4-C5 segment to "implant" the above two kinds of cage for finite element analysis.First of all,select the newly developed mainstream commercial cage(3D ACT cage),by using of computer scanning technology,it can be reversed into STL files that can be operated in the finite element analysis software.Then imitating the shape of 3D ACT cage,the customized cage,which can completely fit the lower endplate of C4 vertebral body and the upper endplate of C5 vertebral body was established in 3-Matic and Magics software and exported as STL file.Finally,the two kinds of cage are introduced into Hypermesh for mesh repair,material attribute assignment and other operations,and then "implanted" into the cervical heterogeneous finite element model.The modeling of customized cage group and 3D ACT cage group was completed.Finally,the preload of 73.6Nm and the additional torque of 1.0Nm were applied to the two groups of cervical finite element models respectively,and then the Von-Mises stress distribution on cage,endplate(C4,C5)and adjacent intervertebral disc(C3/4,C5/6)was analyzed by computer under six working conditions of flexion and extension,left and right lateral flexion,left and right rotation.The stress results were collected and statistically analyzed,and the data differences between the two groups were compared.Results:The results show that the C3～C7 cervical vertebra heterogeneity finite element model is proved to be effective;in the term of cage stress: the stress distribution on customized cage is relatively more uniform,and the maximum stress is significantly lower than that of 3D ACT cage.Under the six working conditions of extension,flexion,right bending,right rotation,left bending and left rotation,the maximum stress of customized cage is253 N and the minimum stress is 0.09 N.Because the middle part of 3D ACT cage is overhead and does not fit well with the endplate,the stress distribution is concentrated on the edge,and the maximum stress is 772 N under six working conditions.in term of endplate stress,the stress distribution of C4 and C5 endplate in customized cage group is relatively more uniform,and the maximum stress is significantly lower than that in3 D ACT cage group.Under six working conditions,the maximum stresses on the C4 and C5 endplates of the customized cage group were 4.72 N and 14 N respectively.The maximum stresses on the C4 and C5 endplates of the 3D ACT cage group were respectively 17.4N and 29N;In the term of stress of adjacent intervertebral disc: there was no significant difference in stress distribution of adjacent intervertebral disc(C3/4,C5/6)between the two groups of models.This shows that while the customized cage reduces the stress of surgical segments,it does not increase the stress compensation of adjacent intervertebral disc(C3/4,C5/6).Conclusion:The C3～C7 cervical vertebra heterogeneity finite element model established in this paper is effective and can be used in three-dimensional finite element analysis.Compared with 3D ACT cage,customized cervical cage has better stress distribution in theory and is more helpful to prevent postoperative subsidence. |
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