The sagittal balance of the cervical spine was evaluated by the C2-C7 Cobb angle (the angle between the vertical line of the lower endplate of C2 and the vertical line of the lower endplate of C7), C2-C7 sagittal vertebral axis (SVA, the horizontal distance between the geometric central plumb line of the C2 vertebral body and the posterior angle of the upper endplate of C7), thoracic inlet angle (TIA, the angle between the T1 midline and the T1 midpoint to the upper of the sternal border), and T1 slope (T1S, the angle between the upper edge of T1 and the horizontal plane).