The Three-dimensional Finite Element Analysis Of The Mechanism Of T1 Slope On Cervical Sagittal Imbalance After Atlantoaxial Fusion

Objective:1.To establish a three-dimensional finite element model of whole cervical spine(C0-T1)with muscles and evaluate its validity,in order to provide necessary conditions for the follow-up study of mechanism of T1 slope on cervical sagittal imbalance after atlantoaxial fusion.2.To explore the impact of T1 slope on the cervical sagittal imbalance after atlantoaxial fusion.3.To identify the role of the abnormal mechanical transmission of cervical spine caused by increasing value of C2 SVA after atlantoaxial fusion on cervical sagittal imbalance.Methods:A 34-year-old healthy male volunteer underwent cervical thin-section CT scans.The raw data of CT scans were stored in Dicom format.Then the CT data was reconstructed reversely with Mimics17 into the 3D cervical point cloud model which was further fitted into NURBS surface model with Geomagic Studio2012.Then NURBS surface model was imported into Hypermesh12 for mesh partitioning,assignment of material property,definition of contact and boundary condition and other operations.Finally,finite element analysis was carried out with ABAQUS 6.12.The range of motion(ROM)in flexion,extension,lateral bending and rotation were compared with the literature datas in order to validate the model.On the basis of the original model above,the T1 slope angle(T1S+10°,T1S-10°)and atlantoaxial fusion angle(C1-2 angle)(C12+10°,C12-10°)were adjusted in order to simulate the conditions that different atlantoaxial angles were performed for patients with different T1 slope angles.Totally nine models were established.The torques of different directions including flexion,extension,left-bending,right-bending,left-torsion and right-torsion were applied respectively to the models.The ROM of each segments,von-Mises stress values of intervertebral discs,annulus fibrosis,ligaments,facet joints and capsules,and the corresponding C2-7 SVA values were obtained in each working conditions.The biomechanical differences of internal structure of each models with different T1 S angle,C1-2 angle and C2-7 SVA value were analyzed and evaluated.Results:1.The model of whole cervical spine with muscles consists of 789024 elements and 285045 nodes.It is similar to the human cervical spine in the appearance.Compared with the literature data,it is found that the ROMs of this model in flexion,extension,lateral bending and rotation are almost in the reference range.2.Increasing or decreasing of T1 S value can cause the according change of C2-7 SVA value.The C2-7 SVA value after atlantoaxial fusion is mainly affected by T1 S.The increasing or decreasing of C2-7 SVA value can lead to the increasing local stress of cervical intervertebral discs,facet joints,joint capsules and ligaments.Conclusions:The normal three-dimensional finite element model of whole cervical spine with muscles established in this study has been validated effectively,and meets the requirements of geometric similarity and mechanical similarity required in finite element analysis,thus can be used for biomechanical analysis of cervical spine.From the imaging perspective,T1 S is the main factor affecting the C2-7 SVA value after atlantoaxial fusion,and from the perspective of disease mechanism,abnormal cervical mechanical transmission caused by C2-7 SVA change is the immanent cause of postoperative cervical sagittal imbalance.The research results remind us that the atlantoaxial fusion angle should be selected individually according to T1 S.