Abstract: Objective　To analyze the biomechanical characteristics of posterior axis different planting methods in upper cervical vertebra under severe osteoporosis (SOP).     Methods    A CT scanning on cervical vertebra was performed on a healthy adult man.  Combined with the finite element pre-processing software, the atlantoaxial instability model (C1~C2) of SOP was stimulated by setting material properties. According to the surgical plan, posterior bilateral atlas pedicle screw (B-C1PS)+ Axis of three different ways of planting nail fixation was established: axis bilateral pedicle screw (B-C2PS, model A) fixation, axis bilateral laminar screw (B-C2TL, model B) fixation, B-C1PS combined with B-C2TL fixation(model C). The range of motion of C1-C2 joint, C1 displacement and C2 screw stress distribution of the three internal fixation methods were analyzed under the bending extension, lateral bending and axial rotation conditions of the finite element model.    Results    The finite element model of C1-C2 instability in the SOP, Model C had the lowest range of motion of C1-C2 joint, C1 displacement in different conditions. Fixed screw under the maximum stress of C2 joint combined fixation was smaller than that of single fixation. The maximum stress of pedicle screw of the axis in the Model C was less than that of Model B under various working conditions, that of pedicle screw in the Model C was less than that of Model A. In Model C, the stress was mainly concentrated at C2PS roots and the joint between atlanto-axial vertebral pedicle screw and screw rod.    Conclusions    The inner fixation stability of Model C is better than that of Model A and Model B under SOP condition. The stress of axis is  dispersed, and the axis screw is less prone to postoperative fatigue loosening and prolapse.

Key words: Atlantoaxial instability, 　Severe osteoporosis, 　Pedicle screw, 　Laminar screw, 　Finite element