Risk Factors for Subaxial Kyphosis After Posterior C1-C2 Fusion for Atlantoaxial Dislocation

Purpose The objective of this study is to evaluate the change of correlation of cervical radiologic parameters after posterior C1-C2 fusion and find the risk factors for postoperative subaxial kyphosis (PSK). Methods Medical records and radiologic data were retrospectively reviewed on patients that had undergone posterior atlantoaxial fusion for atlantoaxial dislocation (AAD) at a single center from January 2014 to December 2022. Previous cervical surgery history, concomitant with basilar invagination, additional fusion extending to occipital or subaxial spine were excluded. Patients’ demographics and radiologic parameters were evaluated for PSK. O-C1, O-C1 range of motion (ROM), C1-C2, O-C2, C2-C7, T1 slope, C2-C7 sagittal vertical axis were measured at preoperative and postoperative one year. Results Mean age was 54.4 years, and 24 of 38 patients were women. RA was the most common pathology, account for 52.6%. Women were a significant risk factor for PSK. Patients presenting decreased O-C1 ROM was 31.6%. The incidence of PSK was 23.7%. Preoperative cervical radiologic parameters showed the sequential linkage of significant correlation from T1 slope to O-C1 angle. However, O-C1 angle was a valuable angle correlated with C2-C7 angle postoperatively. Decreased O-C1 ROM was significantly associated with PSK. The optimal postoperative C1-C2 angle ranged from 15° to 25° Conclusions The correlation between upper cervical spine (O-C2 angle) and subaxial spine (C2-C7 angle) was not changed before and after surgery. After surgery, O-C1 angle was sequentially linked with C2-C7 angle. Women, decreased O-C1 ROM, postoperative C1-C2 angle was significantly associated with PSK.