Finite Element Analysis Of The Relationship Between The Loss Of Correction Of The Vertebral After Removing The Internal Fixation And The "Egg Shell Phenomenon"

Objective: To analyze the relationship between the loss of correction ofthe vertebral after removing the internal fixation and the ‘egg shellphenomenon’ in thoracolumbar vertebral fracture patients who were treated bythe posterior approach operation use the method of finite element analysis.Methods: Select one case of normal adult volunteer, the volunteer ismale and29years old, we do the X-ray examination first to exclude the spinallesions. Then the volunteer’s L1-L3vertebral body was scanned by64-slicespiral CT machine at the condition of: tube voltage of120kV, tube current of200mA, and the scanning conditions are: slice thickness of1mm, theinterlayer spacing of1mm. All data that we got were imported to interactivemedical imaging control system Mimics14.0software in DICOM format, andthen preprocessed to rebuild3D model of the cortical bone and the cancellousbone of L1-L3,(the cortical bone and the cancellous bone of the vertebralwere independent part in the model, discs and ligament tissue were notincluded). After that we have drawn a pedicle screw in size of6.5mm×50mm using the parametric modeling software Creo Parametric2.0that wereproduced by the Parametric Technology Corporation of United States. The3Dmodel of L1and L3vertebral body was assembled with the model of pediclescrew in the3-matic module of Mimics14.0software according to theprinciple of lumbar pedicle screw’s implantation, then we obtained the new3D model of L1-L3that the vertebral body of L1and L3was implanted withpedicle screws. Then the model was meshed in this software, using theBoolean calculation and final got the3D model of L1-L3that the internalfixation was removed. After that, the model was imported to automate reverseengineering software Geomagic Studio12.0in STL format, the intervertebral discs, nucleus, anterior longitudinal ligament, posterior longitudinal ligamentand ligamentum flavum was added according to the normal anatomicalposition using the cut, close, expand, and shift function of the software. Allthese models were meshed in3-matic module of Mimics14.0software. In thesame way, a group of new3D models of L1-L3was rebuilt whose L2vertebral body have ‘egg shell phenomenon’, since the shape of the cavity wasoften irregularly-shaped, we defined it as a sphere, and the variable was thesize and the location of the cavities, they were:1. The cavity located in thecenter of the vertebral body and the diameter of the cavities were the follows:one fifth, two fifth, three fifth, four fifth and equal to(not breakthrough thevertebral endplate) of the vertebral body’s height;2. The cavity’s diameterwas three fifth of the vertebral body’s height and the locations were: A. thecenter of the cavity located in the anterior third point of the vertebral body’ssagittal diameter. B. the center of the cavity located in the postreior third pointof the vertebral body’s sagittal diameter. C. the center of the cavity located inthe superior third point of the vertebral body’s vertical diameter. D. the centerof the cavity located in the inferior third point of the vertebral body’s verticaldiameter. E. the center of the cavity located in the center of the vertebral body.When the models were successfully build, the vertebral body, intervertebraldisc and nucleus were imported to the finite element analysis softwareANSYS Workbench in cdb format, the anterior longitudinal ligament,posterior longitudinal ligament and ligamentum flavum were imported in STLformat. Once imported, we defined the thickness of the anterior longitudinalligament as0.8mm, the thickness of the posterior longitudinal ligament as0.6mm, and the thickness of ligamentum flavum as1.2mm. Then they weremeshed in ANASYS Workbench again. The models of vertebral body, discand nucleus were meshed in hexahedral, the anterior longitudinal ligament,posterior longitudinal ligament and ligamentum flavum models were use thepentahedron combined with the hexahedral to meshed. Normal vertebralmodels were obtained431557nodes and248752units. Set the contactbetween the facet joints of the vertebral body as no separation, the other parts of the contact mode are defined as bonded. Then gave the material propertiesaccording to Tab1.Asummed that the bottom and posterior of L3was fixed,imposed a vertical press of260N and a moment of10N·m on the superiorsurface of L1to simulate the motion of flexion, extension, and lateralcurvature, observe the change of von-Mises of the normal vertebral body andthe “egg shell like” vertebral body under the conditions of above, compare theinfluences by the size and location of the cavity.Results: From the distribution of the von-Mises stress of vertebral body,we can conclude that: A. The von-Mises stress mainly in the posterior of thevertebral end plate and nearby the pedicles, and radiate to the posterior-lateral.B. The von-Mises stress of cortical bone was significantly higher than thestress of the cancellous bone, and the stress was higher in the internal surfacethan that of the outside surface at the edge portion of the cortical bone. C.When the vertebral flexion to one side, both the flexor and extensor sidewould be stress concentration, and the stress in the junction of cortical andcancellous increased significant. D. The von-Mises stress of the cancellousbone under the four movements would be enlarged when there was a cavity inthe vertebral body, and the stress concentration would be more obvious at thejunction of cortical cancellous both on the flexor and extensor side. E. The“egg shell phenomenon” will reduce the of von-Mises stress on the corticalbone, when the cavity became bigger, the von-Mises stress on the corticalbone would be larger, but it will never surpass the von-Mises stress of thenormal vertebral. F. When the size and the location of the cavities were samein the vertebral bodies, there were no other influences to the distribution of thevon-Mises stress in the different movements except that the significantincrease of the stress in the junction of cortical bone and cancellous bone. G.When the sizes of the cavities were same in the vertebral bodies, thecancellous would suffer more large stress nearby the end plate when the cavitywas nearby it too.Conclusion:“Eggshell-like vertebral”will increase the von-Mises stressof the cancellous bone of thevertebral body, while decrease the von-Mises stress of the cortical bone. When the cavity became bigger, the von-Misesstress of the Eggshell-like vertebral”will became larger too. If the cavity isnearby the end plate of the vertebral body, the cancellous would suffer morelarge stress who is nearby the end plate. So the“Eggshell-like vertebral”is arisk factor for the lose of correction after removing the internal fixation.