Influence of the T1-slope on sagittal alignment of the subaxial cervical spine after posterior atlantoaxial fusion in os odontoideum

Abstract Objectives To analyze the effect of T1-slope (T1S) on sagittal alignment of the subaxial cervical spine after posterior C1-C2 fusion. Patients and methods The data of 35 consecutive patients who underwent posterior C1-C2 fusion due to atlantoaxial instability with os odontoideum were retrospectively reviewed. Radiographic parameters, including T1S, C1-C2 angle and C2-C7 angle, were measured before operation and at the final follow-up. Patients were divided into 2 groups: high T1S group and low T1S group. Differences within and between groups in radiographic parameters and the relationships among T1S, C1-C2 angle and C2-C7 angle were analyzed. Results After operation, C1-C2 angle was significantly enlarged (P   0.05). Postoperative C2-C7 angle had a medium positive relation with both preoperative (r = 0.563, P   0.05). However, change of C1-C2 angle demonstrated a medium negative relation with change of C2-C7 angle after operation (r = −0.614, P   0.05). After operation, C1-C2 angles became larger in both groups (all P  Conclusion T1S was a useful parameter to evaluate postoperative alignment of the subaxial cervical spine after posterior C1-C2 fusion. To avoid postoperative misalignment of the subaxial cervical spine, C1-C2 should be fixed in less lordosis, especially in patients with high preoperative T1S.