Biomechanical Analysis Of The Treatment For Denis B Type Burst Fractures On Different Segmental Fusion By Combined Anterior And Posterior Approach

Objective：To develop a three-dimensional finite element model, so as toanalyze biomechanical properties of Denis type B burst fracture treatment ondifferent segmental fusion by combined anterior and posterior approach.Method: Two volunteers with Denis type B burst fracture were operatedwith posterior reduction pedicle screw fixation plus anterior two-segmentfusion surgery and posterior reduction pedicle screw fixation plus anteriorsingle-segment fusion surgery respectively. According to the follow-up oneyear after the surgery, both of them met intervertebral fusion imagingstandards, and then were involved in the study.64-slice spiral CT was used toperform spiral CT scanning and image processing on selected subjects. Themain parameters were as follows: tube voltage of120kV, tube current of200mA, slice thickness of1mm and interlayer spacing of lmm. All imaging datawere saved in DICOM format on DVD discs, imported to interactive medicalimaging control system Mimics software, and then preprocessed to rebuild3Dmodel of T11-L2. Because there was no clear dividing line betweenintervertebral discs and the vertebral body as well as between the superior andinferior facet joints due to irregular vertebrae, in this study a large number ofmanual segmentation methods were used to cross-reference3D model inMimics three-dimensional window against reconstructed3D model in CTworkstation, and these preliminary3D images were imported into Mimics ownmeshing program Magics9.9(MIMICSremesher) via FEA-remesher forsurface mesh division. After FEA module in Mimics optimized the3D modeland divided surface mesh, Mesh tool was used to convert surface mesh modelinto volume mesh model in a preprocessor of the finite element analysissoftware ANSYS. An intervertebral disc model was then developed, and the top and bottom surfaces of the intervetebral disc consisted of cartilageend-plates with1.0mm in thickness; the fiber in annulus fibrosus wasconstructed with truss elements that only bore tensile stress; the fiberstravelled in a scissor-like way, with an angle of averaged25°to40°to thedisc plane. The element type was SOLID92(tetrahedron with10nodes). Themodel was imported into ANSYS for volume mesh, and then re-imported backto Mimics for material properties. The elastic modulus of each part andPoisson’s ratio were selected based on universally accepted literature.According to the structure of the spine, ANSYS was used to assemble eachmodel element, supplemented with the main ligaments such as anteriorlongitudinal ligament, supraspinous ligament, interspinous ligament,ligamentum flavum, capsular ligament, and intertransverse ligament; due to athorough anterior decompression, partial posterior longitudinal ligament wasresected, so all the models were not supplemented with posterior longitudinalligament, and the ligamentous structures mentioned above were simulatedwith spring elements. Ligament elements,3D elements LINK8, bore tensilestress rather than compressive stress. The structure given above wasassembled and given material parameters of the finite element model; aftervarious parts were assembled in accordance with their relative positions inspace, interface was defined according to the actual anatomy. The superior andinferior end-plates were defined as the main planes; the superior and inferiorsurfaces of intervetebral disc were defined as based planes; contact betweenthe superior and inferior facet joints was defined as standard sliding contact,with the friction coefficient of0.2, and the upper surface of facet joint wasselected as the main plane. Thus, a complete three-dimensional finite elementmodel was well developed, and Model1acquired546,609elements and877,237nodes; Model2acquired480,621elements and769,041nodes. Themodels developed were used to simulate two kinds of treatment---combinedanterior-posterior approach with double segmental fusion (Model1) andcombined anterior-posterior approach with single segmental fusion (Model2).The subject treated with single segmental fusion had posterior pedicle screw removed, and according to the method mentioned above we developed3Dfinite element model of T11-L2(Model3), which acquired434,404elementsand688,093nodes. The above-mentioned three models were imported intothe finite element analysis software ANSYS respectively. An axial load260Nand10Nm torque were loaded to simulate the flexion, extension, lateralbending and rotation of the spine. Meantime, the maximum displacement ofthe spine motion and the Von Mises stress of T11-12intervertebral disc wererecorded on each loading.Results: Seen from the vertebral displacement distribution, the spinalmobility of model3increased significantly when compared with model1;model2had a slight increase than model1. When axial load260N and10N·mtorque were loaded, maximal displacement of model2along the x-axis was13.1%and26.0%higher than model1in flexion and extension respectively,model3was310.3%and268.3%higher in flexion and extension respectively;maximal displacement of model2along the y-axis was3.8%and13.6%higher than model1on left and right lateral bending respectively, model3was143.7%and230%higher in left and right lateral bending respectively;maximal displacement of model2along the z-axis was2.3%and23.2%higher than model1in left and right torsion respectively, model3was130.6%and223.4%higher in left and right torsion respectively. The maximal VonMises stress on T11-T12intervertebral disc in model2and model3under theabove loading conditions reduced in varying degrees than model1. Whenaxial load260N and10N·m torque were loaded，the maximal Von Mises stressfor the intervertebral disc of model2was58.2%and54.9%lower than model1in flexion and extension respectively, model3was69.0%and67.8%lowerin flexion and extension respectively; the maximal Von Mises stress for theintervertebral disc of model2was10.1%and20.8%lower than model1onleft and right lateral bending respectively, model3was15.2%and29.3%lower in left and right lateral bending respectively; the maximal Von Misesstress for the intervertebral disc of model2was12.8%and28.9%lower thanmodel1in left and right torsion respectively, model3was19.7%and42.4% lower in left and right torsion respectively.Conclusion: From the biomechanical perspective, ensured the premisesatisfactory intervertebral fusion,The model with combined anterior-posteriorapproach with single segmental fusion after removal the pedicle screw isbetter than the other two models in the aspects of spinal mobility and the stressof intervertebral disc.