| Objective:Anterior cervical corpectomy and fusion(ACCF)is a common operation for the treatment of cervical spondylosis myelopathy.The traditional fusion device used in ACCF is titanium mesh cage,which achieves good stability and fusion rate,but also leads to a serious postoperative complication—subsidence.The subsidence of the titanium mesh is related to the stress shielding effect and its punctate contact with the endplate.In this study,a new personalized ACCF fusion cage was designed using topology optimization techniques to improve the stress shielding effect and the punctate contact problem;the new fusion cage was also compared with the titanium mesh cage using the finite element method to verify whether the biomechanical performance of the new fusion cage achieved the purpose.Methods:A complete 3D model of C3-C7 was reconstructed from CT data of a19-year-old healthy male volunteer.The vertebral body was a nonhomogeneous model with elastic modulus calculated by CT values,the intervertebral disc was a Mooney-Rivlin hyperelastic model,and the ligaments were nonlinear spring elements.The lower surface of C7 was set to be fully constrained in three directions of XYZ,and a preload of 73.6N and moments of 1Nm in six different directions were applied to the upper surface of C3 for finite element analysis of the complete model.The validity was verified by analyzing the activity of each segment under six working conditions of flexion,extension,axial rotation,and lateral bending.To construct the traditional ACCF model,the C5 vertebral body was cut,the anterior longitudinal ligament,posterior longitudinal ligament and intervertebral disc of C4/5 and C5/6 segments were removed,titanium plate,titanium mesh cage and screws were assembled.The titanium mesh cage in the ACCF model was replaced with a cylindrical ring in same size,and both ends of the cylindrical ring were made to fit the endplates by Boolean logic operations.Topology optimization was performed in Hypermesh with the objective of minimizing the weighted compliance under six operating conditions.The material density was adjusted to increase or decrease the retained volume,the regions with higher material density were retained as solid titanium alloy and the other regions were changed to a porous structure with 30%porosity.Finite element analysis of ACCF model with the new fusion cage was performed to compare with the titanium mesh cage model.Results:The peak stress of the optimized cage was reduced by 68.7%-85.3%compared to the titanium mesh cage,and the stress distribution was more uniform,reducing its risk of fracture;the average stress was reduced by78.9%,and the stress on the new cage was reduced,indicating that the stress shielding effect was reduced.On the vertebral endplates in contact with the new fusion cage,the peak stresses were reduced and the average stresses increased,more significantly on the upper endplates of the inferior vertebrae.on the C4 lower endplates,the peak stresses of the new fusion cage model were reduced by 22.2% and the average stresses increased by14.6%;on the C6 upper endplates,the peak stresses of the new fusion cage model were reduced by 54.2% and the average stresses increased by 43.6%,it shows that the stress shielding effect is reduced from another aspect.From the stress cloud of the end plate,it can be seen that the new cage changes the punctate contact to surface contact with the end plate,which further reduces the risk of subsidence.Conclusion:In this study,a new ACCF cage was designed by topology optimization technique and biomechanically compared with the traditional titanium mesh cage by finite element method.The new cage reduced the stress shielding effect and changed the punctate contact between the titanium mesh and the vertebral body to a surface contact,thus reducing the risk of subsidence. |
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