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

Abstract

ObjectivesTo analyze the effect of T1-slope (T1S) on sagittal alignment of the subaxial cervical spine after posterior C1-C2 fusion. Patients and methodsThe 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. ResultsAfter operation, C1-C2 angle was significantly enlarged (P<0.05) while C2-C7 angle reduced (P<0.05). However, significant change was not observed in T1S (P>0.05). Postoperative C2-C7 angle had a medium positive relation with both preoperative (r=0.563, P<0.05) and postoperative T1S (r=0.547, P<0.05). C1-C2 angle was negatively related to C2-C7 angle before operation (r=−0.590, P<0.05) but not after operation (r=−0.125, 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). When the patients were divided into high and low T1S groups, there were significant difference in both preoperative and postoperative C2-C7 angles between 2 groups (all P<0.05) but not in C1-C2 angle, change of C1-C2 angle or change of C2-C7 angle (all P>0.05). After operation, C1-C2 angles became larger in both groups (all P<0.05). Meanwhile, although postoperative C2-C7 angle became smaller than preoperative one in both groups, statistical difference was only observed in high T1S group (P<0.05). ConclusionT1S 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.