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

• Postoperative loss of cervical lordosis after C1-C2 fusion has been revealed.
• High T1-slope was a risk factor to loss of cervical lordosis after C1-C2 fusion.
• C1-C2 should be fixed in less lordosis, especially in patients with high T1-slope.

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.