Design,Manufacture,and Biomechanical Research Of A 3D Printed Customized Cervical Intervertebral Fusion Cage

In recent years,3D printing technology has developed rapidly and is now widely used in the medical field.In the field of orthopedics,the use of 3D printing technology allows for individual customized implant designs,such as intervertebral fusion cage.Interbody fusion cage is commonly used in spinal surgery to open up the vertebral space,protect the interbody height and maintain vertebral stability.Traditional intervertebral fusion cages have a standardized design that cannot be adapted to the individual patient and often result in stress concentrations at the endplate interface due to a mismatch between the shape and the vertebral body,which can lead to postoperative complications such as displacement and subsidence of the intervertebral fusion cage.Therefore,we propose the use of 3D printing technology for individualized intervertebral fusion cage to achieve macroscopic shape bionics,however,solid intervertebral fusion cage still suffers from a high modulus of elasticity,which tends to cause stress shielding and thus aggravates the risk of subsidence of the cage.To solve the scientific problem,we proposed the innovative scientific hypothesis "3D printing topology optimization combined with personalized design and progressive porosity technology for intervertebral fusion cage can reduce the occurrence of postoperative complications".The proposed development of a cervical intervertebral fusion cage is based on topology optimization and individual design.Finite element analysis topology optimization is a design method that optimizes the material distribution in the target area and removes redundant structures,optimizing the overall structure of the cervical intervertebral fusion,reducing the average elastic modulus of the fusion and reducing the stress shielding phenomenon.The combination of this with the tapering porous structure design also improves the stability and rigidity of the fusion cage,while reducing its weight and increasing its surface area,improving the effectiveness and the safety of the procedure.This study proposes to design a customized intervertebral fusion device with bionic properties and to develop a new intervertebral fusion cage with improved postoperative complications such as subsidence and displacement based on a personalized design concept by combining finite element analysis and topology optimization techniques.Assessing its biomechanical advantages and providing new design ideas for interbody fusion devices to reduce postoperative subsidence.During the development process,the volunteers’ cervical spine was first scanned with CT data and a 3D reconstruction of the volunteers’ cervical spine was performed.The structures associated with the cervical spine such as the intervertebral discs,ligaments and cartilage were established.An accurate finite element analysis model of the complete cervical spine was established using a non-homogeneous assignment method to build a surgical model for anterior cervical discectomy and fusion(ACDF)based on a healthy model.The solid conventional intervertebral fusion cage was scanned for inverse reconstruction,a personalized intervertebral fusion was created based on the same outer contour fitted to the upper and lower vertebrae,and a finite element analysis biomechanical comparison of the conventional intervertebral fusion cage and the personalized customized intervertebral fusion cage was carried out to determine the advantages of personalization.Topology optimization on an individual basis,with collaborative optimization in six operating conditions: flexion,extension,left bending,right bending,left rotation and right rotation.The customized intervertebral fusion cage was redistributed according to the mechanical load-bearing situation,retaining the high-density areas of primary stress and removing the lowdensity areas of secondary stress to obtain a topologically optimized intervertebral fusion cage.The topology optimization results were post-processed and the results were biomechanically analyzed using the ACDF surgical finite element analysis model.The suitable topology optimization results in a gradient porosity design assigned to the intervertebral fusion cage using cell density and finite element analysis of the cage with a gradient porosity structure.The designed new intervertebral fusion cage was manufactured in 3D solid form using an EBM metal printer,and the new cage was tested and evaluated using a scanning electron microscope and a universal mechanical testing machine.The study results showed that by establishing an ACDF surgical inhomogeneous finite element model,the designed customized intervertebral fusion cage and the scanned reconstructed commercial intervertebral fusion cage were implanted into the model respectively.After finite element analysis,it was found that the peak stress of the conventional commercial intervertebral fusion itself reached 709.3 MPa in the lateral bending condition,which was much larger than the 175.3 MPa on the customized intervertebral fusion cage.The customized design cage of the intervertebral fusion was less stressful on the surface and the adjacent interface was also less stressed than with a conventional intervertebral fusion cage.After topological optimization of the intervertebral fusion cage designed with the individual concept,the fusion cage model was obtained for different iteration groups and validated in the ACDF surgical model,which showed that the stresses on the intervertebral fusion device in the three iteration groups were minimal under different conditions.Based on the topological optimization results,the optimized intervertebral fusion cage was treated using a gradual porosity approach and loaded into the ACDF surgical model for finite element analysis,which revealed a significant improvement in the stresses at the adjacent vertebral interfaces.In addition,the volume of the intervertebral fusion cage has been reduced by 61% compared to the original intervertebral fusion and the surface area has been increased by 210%,with its porous structure improving contact with the bone cells and enhancing bone ingrowth.The simulated design of the new intervertebral fusion was realized using a 3D metal printer and the gradual porous structure of the intervertebral fusion was observed using scanning electron microscopy without distortion due to EBM printing,and in vitro mechanical compression tests also confirmed the corresponding strength of the fusion.In summary,this technique demonstrates the biomechanical performance of a topologically optimized,individually designed,tapering porous intervertebral fusion device.In addition to having a lighter mass and larger surface area,it can improve the biomechanical environment,reduce the stress shielding effect with the vertebral body and reduce the risk of postoperative sinking,displacement,and bone non-union of the intervertebral fusion cage,which has a wide range of clinical applications.This subject provides the basis and novel ideas for the personalization of orthopedic implants.