Development And Biomechanical Study Of 3D Printing Porous Structure Expandable Interbody Fusion Cage

1 Objective:To develop a highly adjustable lumbar interbody fusion cage that comply with biomechanics of human via selective laser melting(SLM)3D printing technology.2 Method:2.1 Development and mechanical research of cage:developing expandible and porous titanium cage of lumbar basing on SLM 3D printing technology.Material testing machine MTS model E45 that“fits-all”materials was used to study the mechanics of the interbody fusion cage(Axial compression performance test),and the results have been analyzed statistically.2.2 Finite element analysis of structure and materials of fusion cage:using UG three-dimensional CAD modeling,Ansys LS-Dyna general nonlinear finite element calculation software to establish four cages including:Porous Cage,titanium alloy TC4 Cage,PEEK Cage and traditional static PEEK-Cage.All the cages were established and characterized with different properties including elastic modulus,Poisson’s ratio,yield stress,failure stress,yield strength limit,elongation,density,etc.Subsequently,the cages were used to carry out finite element mechanical analysis of fusion cage structure and materials,respectively.2.3 Finite element analysis of simulated lumbar surgery:the DICOM format data of normal lumbar spine CT acquired from a healthy male volunteer in our hospital were collected and imported into Mimics Medical 20.0 medical 3D reconstruction software.Reverse engineering reconstruction with Geomagic2017 software and analysis operation by Ansys Workbench18.2 software were performed to establish a normal three-dimensional finite element model of lumbar 3～4segments,and the validity of the model was validated.The lumbar surgery was further simulated by finite element method,and porous stretchable Cage and traditional static PEEK-Cage were assembled with minimally invasive lumbar surgery to simulate the immediate physiological activities of flexion and extension,left and right flexion,left and right rotation,and to analyze the finite element mechanical stability and stress changes of Cage in lumbar surgery.3 Results:3.1 Lumber cage with porous structure was developed successfully with SLM printing technology.Height of this cage can be adjusted from 9to 13mm.The porous part of the endplate contacting parts is optimized and printed by porous titanium alloy structure.Also,the elastic modulus is suitable(about 1.47Gpa),which can reduce the stress shielding effect in theory.In the axial compression test of the Cage,the yield and ultimate load of the porous extendable Cage group were basically the same as the traditional PEEK-Cage group(P>0.05),which met the mechanical needs of the intervertebral implants,and the compression stiffness of the porous expandable Cage group was lower than that of the traditional PEEK-Cage group(P<0.01),indicating that the general elastic modulus of the fusion cage was smaller.3.2 Finite element analysis of Cage structure and materials:observe the curve of KE/IE with time,the ratio is less than 5%,which meets the quasi-static requirements.The maximum failure load of porous expandable Cage group is slightly higher than that of traditional static PEEK group(porous expandable Cage:12.2×10~3N,traditional PEEK:10.97×10~3N).From the stress cloud diagram of the interface of Cage endplate,there is no obvious stress concentration on the interface of porous expandable Cage endplate,and the main stress structure is the solid titanium alloy frame around the porous.The designed porous expandable Cage meets the mechanical requirements of intervertebral implants.3.3 The three-dimensional finite element model of human lumbar segment 3～4 was successfully established,and lumbar mobility under different load conditions were within the range reported in literatures before.On this basis,porous expandable Cage and traditional static PEEK-Cage of the same size were assembled respectively to mimic minimally invasive spinal surgery that performed left articular process plasty and posterior nail rod internal fixation.Three-dimensional finite element analysis:under six different working conditions of flexion and extension,left and right lateral bending and left and right rotation,the maximum stress values of porous expandable Cage group were 39.97,55.93,59.28,63.34,57.94,49.32MPa respectively,while those of traditional PEEK-Cage were 47.36,35.05,38.62,29.96,73.26 and75.68MPa,respectively,all of which were less than the yield stress of the two fusion cage materials.At the same time,the cloud image analysis of the contact endplate of the two fusion cages shows that the blue-green stress distribution of the porous expandable Cage is similar to that of the traditional PEEK-Cage,and the maximum stress of the porous expandable Cage contact endplate is lower than that of the traditional PEEK-Cage group.From the analysis of the range of motion and displacement of lumbar vertebrae,the range of motion of porous expandable Cage under six different working conditions was 0.33,0.30,0.61,0.57,0.62,0.67mm respectively,while that of traditional PEEK-Cage was 0.59,0.33,0.66,0.61,0.77 and 0.81mm,respectively.The range of motion of the vertebral body in the porous expandable Cage group was lower than that in the traditional PEEK-Cage group,indicating that the stability of the porous expandable Cage in the vertebral fusion segment was better than traditional Cage.4 Conclusion:Compared with PEEK-Cage,the Cage based on SLM can reduce the stress shielding effect,disperse the surface stress of endplate and improve the stability of operation segment,to meet the biomechanical needs of spinal interbody implants.