| [Backgrounds]Structured upper thoracic curve had a great impact on shoulder balance for Lenke2type adolescent idiopathic scoliotic patients. An important preoperative decision waswhether fusion and instrumentation of both curves was necessary, or whether, after fusionand instrumentation of the main curve only, the upper thoracic curve would spontaneouslycorrect itself in compensation for the surgical straightening of the lower curve.It wasimportant that we chosed the proper upper thoracic curve fusion strategy to maintainbalanced shoulder and improve patient satisfaction. King and his colleagues ascribed thistype to King Type V curves recommended fusion of both the upper and lower thoraciccurves. Puno also thought that it was necessary to fusion both thoracic curves to correct T1tilt and achieve good shoulder balance.In addition, Winter reported that selective mainthoracic curve fusion could carry out if preoperative right shoulder was higher or level.However, overcorrection of the MT curve beyond the flexibility of the PT curve resulted inspinal imbalance and shoulder imbalance. Suk et al concluded that if the preoperativeupper thoracic curve was＞25°and the right shoulder was higher, selective MT curvefusion could carry out and postoperative upper thoracic curve did notprogression.Furthermore, Kuklo et al reported a slightly higher degree of a spontaneouscorrection at the upper thoracic curve with anterior selective main thoracic correction thanposterior correction.As a new technique of digital simulation,finite element method had been a hot topicof scoliotic biomechanics recently．Finite element simulation of correction procedure hadbeen focused on posterior90°rod rotation maneuver currently,few simulation of anteriorcorrection or comparison of posterior and anterior correction was reported．[Objectives]A series of related CAE software were used to establish three dimensional finiteelement models of the Lenke2type idiopathic scoliotic spine based on quantitativecomputed tomography scans． After parameters optimization and validation, thethree-dimensional finite element model was used to simulate different anterior andposterior correction strategies. Through the simulation,we want to investigate correctioneffectiveness with different surgical methods and provide a theoretical basis of biomechanical characteristics．[Methods]1．Establishment of the three-dimensional finite element modelA16-year-old male Lenke2type idiopathic scoliotic patient was included asvolunteer for the current study. CT transverse scanning in supine position was done fromC7to caudal end of the spine in1mm layer interval. All CT images were imported intoMimics10.0to form a3-Dimension geometric model with surface mesh by the followingsteps: orientation,division,erase,draw,erode,dilate,boolen operation,define polylineand remesh．And then the geometric model was further delivered to the software Ansys11.0to build3D finite element idiopathic scoliosis model by mesh partition and qualitycontrol．A variety of material parameters were given to different mesh according toreferences．2．Personalization of the mechanical property and validation of optimized finite elementmodelThe personalization of the mechanical properties is done using the flexible testsroutinely done prior to the surgery based on preoperative anteroposterior radiographs andsupine side-bending radiographs. And using the orthogonal experimental design analysis ofthree factors and three levels of disc material property, the3D finite element idiopathicscoliosis model was optimized and achieved the biomechanical propertyindividually.Geometric shape was compared between supine X-ray films and finiteelement simulation. Bending test were simulated．Different segments were extracted fromthe whole finite element model,and were respectively constrained and loaded referring tohistorical biomechanical specimen in vitro studies．3．Simulation of different surgical strategies in Lenke2type AIS modelThe finite element model of internal fixation devices was developed．According todifferent surgical approach and fusion level,we designed and simulated five surgicalstrategies with the model of scoliosis: anterior selective main thoracic curve fusion,posterior selective main thoracic curve fusion, Partial fusion the proximal thoracic curvesto T3or T4and complete fusion the proximal thoracic curves to T2. Posterior90°derotation maneuver and anterior simple segmental convex compression weresimulated. Different vertebral displacement, as well as angle changes and shoulder balanceparameters were compared among the protocols of different surgical approach and fusionlevel． [Results]1.Three-dimensional finite element model of Lenke2type idiopathic scoliosis was builtsuccessfully, including C7,all thoraco-lumbar-sacral spine and thoracic cage,using4meshtypes and13kinds of material parameters,and consisting of638505physical elements,32152shell elements and634Link elements．The three-dimensional Finite ElementModel of the scoliotic spine showed good geometric similarity with supine X-ray．2. Orthogonal experiment results showed that disc property in the proximal thoracicsegments, main thoracic segments, lumbar segments were8,8,0.2respectively. And thatwas the best combinations,which mimimized the difference of the model and the actualindividual. Compared the Cobb angles of optimized model and actual X-ray film,thelargest difference was2°.Compared the center of mass deviaed from central sacral verticalline between X-Ray and optimized model,P＞0.05,which considered no differencesbetween them．Chose the three segmants including T10-T11,T12-L2and L4-L5tocompare with related results of biomechanics empirical study,the motion of the model wasin the boundary of related results．3. Coronary upper thoracic curve was corrected to21.5(44.8%),26.5(32.1%),28.1(27.9%),34.1(12.5%),32(17.9%)respectively for all five surgical strategies used inthe simulation. Physiological saggital configuration was maintained． Postoperativeradiographic shoulder balance parameters were increased among all five surgical protocols.Coracoid height difference＞9mm, clavical angle＞2.5°and clavicular tilt angle difference＞4.5°for all five surgical strategies except the strategy A(posterior fusion to T2).[Conclusions]1. Based on CT images, the three-dimensional finite element model of Lenke2typeidiopathic scoliosis were well estalblished and validated.2. The validity of the optimized finite element model was verified successfully, so it wasproved to be able to represent the biomechanical behaviour of the actual scoliotic spine．3. The established lenke2type AIS model was successfully used to simulate correctionprocedures including posterior90°derotation maneuver and anterior simple segmentalconvex compression for the first time．Finite element method may provide theoretical basisand guidance for the surgical planning and effectiveness evaluation．4. For the lenke2type AIS model with structured proximal thoracic curve and preoperativeleft elevated shoulder, Simulation of Posterior correction and fusion to T2including boththoracic curve achieved excellent three-dimensional corrective results of both the proximal and main thoracic curves as well as balanced shoulders. Partial fusion the proximalthoracic curves to T3or T4resulted in inferior corrective rate of PT curve and mild ormoderate shoulder imbalance. Simulation of anteror or posterior selective main thoraciccurve fusion had very low spontaneous PT corrective rate and could not improve thealignment significantly. Shoulder imbalance also deteriorated.So selective main thoraciccurve fusion maybe not a good choice for double thoracic curve with preoperative leftelevated shoulder.
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