Construction Of 3D Finite Element Model Of Lumbar Short Segment Pedicle Screw Instrumentation And Stress Analysis

BackgroudsPedicle screw fixation system strongly fixed anterior,axial and posterior column at the same time,strengthen support and anti-torsion stiffness,and achieve better fusion rate and clinical results.However,with the extensive application of the technology,its operation is common case of failure,such as fracture fixation,adjacent segment degeneration,and the most common complication is fracture fixation.In recent years,application of finite element method in spinal biomechanics research is increasingly wide and deep.Among these,useing of bio-mechanical knowledge,a comprehensive analysis of the role of theory and stress distribution of various internal fixation devices have great significance to the correct selection of surgical methods, to rational design and use of internal fixation devices,to achieve the best orthopedic and fixed effects,to reduce the operation failed rate and the occurrence of complications.This subject is subvented by the Technology Project of Guangdong Province in 2007,"The R & D and applied research of double U-shaped biology flexible memory alloy rod fixation system "(Item Number:2007B031003010),is a subset of the.Technology Project in fact,and provides the objectives of improvements and evaluation of the control targets for the development of new types of flexible pedicle screw fixation system.Objectives1.To establish three-dimensional finite element model of short-segment lumbar pedicle screw fixation system,2.To provide three-dimensional finite element model for short-segment lumbar pedicle screw fixation system,on the basis analysis stress distribution of screws and rods under different loading.3.To provides the objectives of improvements and evaluation of the control targets for the development of new types of flexible pedicle screw fixation system.Materials and Methods1 Establish 3D finite element model of lumbar short segment pedicle screw instrumentation1.1 Image data:a selection of adult volunteers,by X-ray examination to exclude spinal disease,PHILIPS Brilliance 64-slice spiral CT scaned him by continuous level, scan range from L₁ to the greater trochanter of femur,image processed and stored in the CT workstation.1.2 Computer hardware:AMD Dual-Core 5000+ Hyper-Threading CPU,4G DDR800 RAM,6800 GT graphics accelerator card dual-128M PCIE.1.3 Software:Mimics 11.11,Simpleware2.1,Abaqus6.51.1.4 Modeling Methods:The geometrical model was created by Mimics 11.1 based on CT data of L_3-5 segment.The models were imported into the Simpleware 2.1 and pedicle screw fixation system was shifted in with STL format to establish 3D finite element models of posterior lumber internal fixation.The 3D finite element models were imported into Abaqus 6.5 and analyzed.At the same time,according to the geometry and mechanical parameters of screw and rod,established three-dimensional finite element model of lumbar pedicle screw fixation system.2 Loading and Calculation2.1 Loading method:(1) fixed the lower surface of L₅,500 N pressure was loaded on the upper surface of L₃,and this load evenly distributed on the surface of each node;(2) At the same time,15 N·m moments was loaded to simulate lumbar anteflexion,extension,lateral bending and rotation(Considering about the symmetry of pedicle screw fixation,only loaded left bending and right rotation).The stress of the pedicle screws was recorded. 2.2 The results of the qualitative observation:Showed with the stress distribution diagram.2.3 Design of calculation:calculated screws and rods the average Von Mises stress:screws were evenly divided into 3 parts according to the their length,each uniformly selected 32 nodes,and then loaded in the above-mentioned circumstancees, calculated each node stress,calculated the average as the stress of the part;connect rods were evenly divided into 2 parts,each evenly selected 32 nodes,each node used the same method to calculate stress,used the average as the stress of the connecting rods.3 Statistical MethodsAll statistical analysis used SPSS 16.0 statistical software to deal with. Comparison of stress at different parts and under different moves used variance analysis of factorial design,if the overall comparison P＜0.05,the further multiple comparisons(Homogeneity of variance used the SNK method,while variance arrhythmia used the Dunnett T3 method).Test standard:α= 0.05.Results1.Mesh results of 3D finite element model of the normal lumbar spinal:The model builded by mimics,simpleware and abaqus has a excellent geometric similarity.And reproduce the L₃-L₅ segmental anatomy shape,the overall visual display surface without too much simplification.2.Stress distribution of pedicle screw fixation system at different parts and under different moves:The stress of axial direction in various part of pedicle screw instrumentation is far smaller than the stress of flexion,extension,lateral bending, and rotation forces.The stress of axial direction mainly concentrates on the conjunction of screw-rod,while the stress of flexion,extension,lateral bending,and rotation forces mainly concentrates on the rod.(Although the average stress of posterior segment of screw is less than that of connecting rod,yet the stress peak in the only screw nail is similar to connecting rod,which is also great showed with the stress distribution diagram.)Conclusions 1.The method of this study is that:One common volunteer was chosen,who was scanned by multi-slices computerized tomography(MSCT),then the 2D image data in format of DICOM was obtained,and then,from this data,a three-dimensional image about lumbar spinal was reconstructed with the software of Mimics.Finally this 3D image was modified and smoothened by the software of Simpleware and the 3D solid modeling of lumbar spinal was reconstructed successfully.This method is very quick,convenient,high efficient,precise.2.The experiment simulated L_4-5 pedicle screws fixations by three-dimensional finite element model,close to the lumbar rear decompression,internal fixation without bone graft or with posterolateral bone graft has not fused.The lumbar spine loads and torque loads are near normal physiological state of the waist level.The results show that:the stress of axial direction in various part of pedicle screw instrumentation is far smaller than the stress of flexion,extension,lateral bending,and rotation forces. After operation,fixation system should be made in the body to avoid stress concentration and lead to fracture.So after operation,bed rest and wearing braces, patients avoid spinal flexion,extension,lateral bending and rotation movements, mainly load from the vertical compression.The stress of axial direction mainly concentrates on the conjunction of screw-rod,which is fully consistent with the most common fracture site of the most commonly used,the traditional rigid pedicle screw spinal fixation system On clinical.At present,for more study of screw geometry, there is cylindrical,conical and Gourd-shaped.But no matter how the design of the pedicle screw system,the stress of axial direction mainly concentrates on the conjunction of screw-rod.so the conjunction of screw-rod can not be too small in diameter.However,the size of the human body pedicle diameter is limited,when the screw cross-sectional area accounted for 90 percent,then increase the diameter of the screw,there is no increase in fixation strength but easy to make pedicle burst fractures.So a new pedicle screw fixation system,only the stress distribution,no significant high concentration of the stress situation,at least for the screws which are unable to strengthen,can be effective to avoid pedicle screw fixation system fracture that the most common complications.This is one of the objectives of improvements and evaluation indicators for the development of new types of flexible pedicle screw fixation system.3.The 3D visible geometry model and 3D finite element skeleton model could be constructed from slice section images with Mimics,Simpleware and Abaqus software.The finite element model simulate morph and mechanics performance of human body reliably.It is a good method to operate vitodynamics experimentation with digitizated mechanics skeleton model.The finite element model could substitute human body partly,it has the advantages of affluence information,purity,low-cost, low time consuming,high efficiency,precise simulation,favorable repeatability.Also this method can complement with biomechanics in vivo experiments each other.4.Based on this 3D finite-elemental model of short-segment lumbar pedicle screw fixation system,many relative studies can be expanded in the future.For example,in the original model,easily analog newly developed U-shaped memory alloy stick biology flexible fixation system,and further investigate the evaluation by compare the stress distribution of the traditional rigid pedicle screw fixation system and newly developed flexible fixation system.Same method can also be used to compare the stress of its adjacent segment of traditional rigid pedicle screw fixation system and newly developed flexible fixation system,and further analysis and evaluate which is better for avoiding degeneration of adjacent segment.In short the traditional rigid three-dimensional finite element model and stress analysis for the further study of the latter has laid a good foundation.5.The disadvantage of this test is that all the biomaterials were supposed to homogeny,continuous and isotropy.But that is different from the fact.Because this test only in the computer simulate simple,basic movements of lumbar spine,whether its geometric similarity and mechanical similarity,or border constraints and loading, there are all inevitable differences from the actual situation;and the accuracy of finite element model depends on the number of units selected,more the number of units selected more the model accurate,and vice versa,so the purpose of test is to provide biomechanics and theoretical foundation for clinical.The actual clinical value of the evaluation of the traditional rigid pedicle screw fixation system and newly developed flexible fixation system,still need physical specimens biomechanical testing and eventually clinical validation.6.The finite-element method and the method of biomaterial test of the specimen have respectively advantages and disadvantages.The two methods should make up each other,then the more scientific and reasonable results can be obtained finally.