寰枢椎融合术后颈椎矢状面失衡是临床常见问题，目前C1-C2融合角度被认为是影响术后颈椎矢状面平衡的关键因素。然而我们前期发现，部分C1-C2融合角度理想的患者，术后仍出现了颈椎矢状面失衡，胸1斜度（T1S）在维持术后颈椎矢状面平衡方面起着决定性作用，T1S越大越容易出现颈椎矢状面失衡。因此我们推论，寰枢椎融合术后，C1-C2角度发生改变，患者为保持平视，颈椎后方肌肉韧带复合体做功增多以维持头部直立姿势，而大T1S容易导致复合体结构失代偿，C2矢状垂直轴（SVA）逐渐增大，导致前方椎间盘压力增大，颈椎退变加速，最终导致颈椎矢状面失衡。因此，我们将通过生物力学研究，分析寰枢椎融合术后不同T1S对颈椎各结构的生物力学影响，并记录不同C2SVA时各结构的应力值，揭示T1S在寰枢椎融合术后颈椎矢状面失衡中的生物力学机制，为临床根据T1S选择最佳C1-C2融合角度、防止术后颈椎矢状面失衡提供理论依据。

Cervical sagittal imbalance after atlantoaxial fusion is not an uncommon clinical problem. C1-C2 fusion angle is considered as a key factor affecting postoperative cervical sagittal balance. However, we found that some patients developed postoperative sagittal imbalance even thought the C1-C2 angle was fixed in an optimal position. Furthermore, T1 slope (T1S) was found to be the key to regular postoperative sagittal balance. The more the T1S was, the more sagittal imbalance was prone to develop. Therefore, we concluded that C1-C2 angle was changed after atlantoaxial fusion. As a result, posterior cervical muscle ligament complex worked hard to maintain an upright head. Large T1S was prone to lead to decompensation of the complex. Then, the C2 sagittal vertical axis（SVA）was gradually enlarged, increasing the intervertebral disc pressure, accelerating the degeneration of the cervical spine, and finally leading to cervical sagittal imbalance. Therefore, we would perform a biomechanical study to analyze the influence of different T1S on cervical spine and record the stresses with different C2SVA, and finally reveal the mechanism of T1S on cervical sagittal imbalance after atlantoaxial fusion. This study would provide a theoretical basis for choosing the best C1-C2 fusion angle according to T1S to prevent postoperative cervical sagittal imbalance.