MINI: This study validated that correction of clivo-axial angle will influence the subaxial cervical lordosis (ΔCXA = -ΔCL) of atlantoaxial dislocation (AAD) patients. This equation can serve as a quantitative reference for preoperative planning and intraoperative refining of the correction of cervical sagittal deformity in AAD. A mechanism-based reasoning and retrospective cohort study. To establish a quantitative relationship between the change of clivo-axial angle (CXA) and the change of subaxial cervical lordosis (CL) in correction surgery of Atlantoaxial Dislocation (AAD). The evolving understanding of mechanism has dramatically improved the treatment modality of AAD. Cervical sagittal alignment is another important aspect of the surgery, in addition to reduction of odontoid process. However, a quantitative reference for correction surgery has not been established. Frankfort-axial angle (FXA) was introduced. Based on two assumptions, (1) sagittal alignment of the spine aims at keeping horizontal gaze and (2) deformities at craniovertebral junction makes little impact on slope of T1 vertebra, we deduced that the change of CXA equaled change of CL (ΔCXA = -ΔCL). We retrospectively reviewed our case cohort to validate this finding with linear regression analysis. Sixteen cases (8 male and 8 female, mean age 40.4±12.5 year old) were included. Liner fitting equation for ΔFXA and ΔCXA is y=1.005x (coefficient of determination, R=0.966; Significance of the estimated coefficients P<0.001, t-statistics) and that for ΔCL and ΔCXA is y=-1.023x (R=0.976, P<0.001). These results support our deduction that ΔCXA = -ΔCL, which can be used as a guidance of quantitative correction of sagittal deformity in AAD. Correction of CXA will influence the subaxial cervical lordosis (ΔCXA = -ΔCL) of AAD patients. This equation can serve as a quantitative reference for preoperative planning and intraoperative refining of the correction of cervical sagittal deformity in AAD. 4.