Establishment Of Finite Element Model Of Lenke1BN Idiopathic Scoliosis And Posterior Three-dimensional Correction Biomechanical Study

In current study,we established,based on CT images,and validated a complete three-dimensional finite element model of Lenke1BN idiopathic scoliosis(IS),including all thoraco-lumbar-sacral vertebrae and thoracic cage,using computer aided engineering(CAE) softwares. On the basis of above,we simulated posterior three-dimensional correction surgery using this IS finite element model to investigate correction effectiveness with different lowest instrumented vertebra.Chapter One Establishment of Three-dimensional Finite Element Model of Lenke1BN Idiopathic ScoliosisObjective Using CAE softwares,to build three-dimensional finite element model of Lenke1BN idiopathic scoliosis based on CT images.Methods A 18-year-old female Lenke1BN idiopathic scoliosis patient was included as volunteer for current study.CT transverse scanning in supine position was done from T₁ to caudal end in 1mm layer interval,to obtain 539 CT dicom images.All CT images were imported into Mimics 10.01 to form qualified IS three-dimensional geometric model after geometry clean,including all thoraco-lumbar-sacral vertebrae and thoracic cage,which was further delivered to HypherMesh 7.0 to build 3d finite element IS model by mesh partition and quality control.A variety of material parameters were given to different mesh according to references,and 100N axial tension on top surface of T₁ was loaded for preliminary loading calculation using the finite element IS model,to check model quality.Results A three-dimensional finite element model of Lenke1BN idiopathic scoliosis was built successfully,including all thoracolumbar -sacral spine and thoracic cage,using 4 mesh types and 14 kinds of material parameters,in consist of 341228 nodes,1409929 tetrahedron elements,163132 shell elements,715 cable elements and 149 rod elements.Preliminary loading calculation was completed smoothly.Conclusions A three-dimensional finite element model of Lenke1BN IS in details,was built successfully based on individual CT images.Excellent preliminary loading calculation results using the FE model,proved mesh well-partitioned.Chapter Two Validation of Three-dimensional Finite Element Model of Lenke1BN Idiopathic ScoliosisObjective To validate 3-dimensional finite element model of Lenke1BN IS built in chapter one,by contrast with in vitro studies.Methods(1) Left and right Bending test validation:To measure the vertical distances between T₁ and center sacral vertical line(CSVL) on left and right supine bending X-ray film,finite element IS model was constrained and loaded to simulate supine bending test,with T₁ to be moved left and right the same distances to CSVL.The convex bending Cobb's angles of proximal thoracic curve(PT),main thoracic curve(MT) and lumbar curve(L),and the average distance of T₁-L₅ to CSVL were compared between bending X-ray films and finite element simulation. (2) Erect-supine test validation:Finite element model were simulated erectly by applying corresponding downward forces to every segment for gravity and muscle actions according to references.Erect and supine Cobb's angles of PT,MT and L were measured and compared on A-P X-ray films and finite element simulation.(3) Subsection validation: Segment T_10-11,T₁₁-L₁ and L₁-S₁ were extracted from the whole finite element model,and the three segments were respectively constrained and loaded referring to historical specimen biomechanical in vitro studies.Results(1) The convex bending Cobb's angles of PT,MT and L curve on X-ray films and finite element simulation were 14°,26°,8°and 15°,24°,6°respectively.The error of finite element simulation was 8.3%. The average vertical distance of T₁-L₅ to CSVL on left and right supine bending X-ray film was 9.83±7.08cm and 10.15±7.34cm,with no significant difference comparing to finite element model simulation of 9.55±7.04cm and 10.02±7.35cm correspondingly(P>0.05).(2) Erect Cobb's angles of PT,MT and L were 37°,50°,24°and 33°,51°,24°on X-ray films and by finite element simulation.Supine Cobb's angles of PT, MT and L were 29°,43°,22°and 27°,42°,22°on X-ray films and by finite element simulation.The average error of finite element simulation was 3.9%.(3) The segment simulation results were similar to their references respectively.Conclusions The three-dimensional finite element model of Lenke 1BN idiopathic scoliosis were well validated by geometry appearance, left and right supine bending test,erect-spuine test and segment validation,which was qualified for further biomechanical simulation study.Chapter Three Three-dimensional Finite Element Simulation of Posterior Surgical Correction of Lenke1BN idiopathic scoiliosisObjective To simulate posterior correction surgery using Lenke 1BN IS finite element model and investigate correction effectiveness with different lowest instrumented vertebra(LIV).Methods Posterior pedicle screws on concave side and pre-bent rod placed were placed on IS finite element model,constraints and loadings were applied at the same time as follows:sacrum constrained horizontally,corresponding downward forces applied to every segment for gravity and muscle actions according to references,10Nm torsion moment against convex applied to apical zone,and proper torsion moment applied to rod to rotate 90 degrees backward.The upper instrumented vertebra(UIV),was selected to T₄ and LIV down to T₁₂ (NV),L₁(SV) and L₂(SV+1) respectively,with comparison of correction effectnesses among the three LIV choices.Results After simulations of posterior correction,Cobb's angles of proximal curve,main thoracic curve,lumbar curve and T_5-12 in sagittal plane,fusion down to T₁₂,L₁ and L₂ were 7.1°,7.4°,9.2°,21.3°;6.4°6.8°,8.3°,20.7°and 6.5°,7.2°,8.6°,20.5°respectively.Correction rates were insignificantly different among the three LIV choices.Conclusion Selective thoracic curve fusion can lead to satisfactory spontaneous correction of lumbar compensatory curve for Lenke1BNIS. When all pedicle screws instrumentation and direct vertebra rotation of apical zone are used,fusion extended down to neutral vertebra(NV) is enough,with less segments fused than traditionally down to stable vertebra(SV).