Sagittal alignment correlates with the C1-C2 fixation angle and functional outcome after posterior atlantoaxial fixation for traumatic atlantoaxial instability

Abstract

The C1-C2 angle has been shown to correlate with subaxial alignment under various conditions. The aim of this study was to evaluate the correlation between the C1-C2 fixation angle and subaxial sagittal alignment as well as the impact of the sagittal vertical axis (SVA) on functional outcomes in traumatic atlantoaxial (A-A) instabilities.The data of 36 patients who underwent posterior C1-C2 fixation for traumatic A-A instability between December 2005 and September 2015 were retrospectively reviewed. Radiographic parameters, including the C1-C2 angle, occipitocervical angle, C2-C7 angle, and C2-C7 SVA, were measured before surgery and at 1-year follow-up. Clinical outcomes were measured using the visual analogue scale (VAS) and Neck Disability Index (NDI). The preoperative and postoperative relationships between parameters were analyzed.In preoperative and postoperative radiographs, the C1-C2 angle correlated with the C2-C7 angle (r = −0.347, p = 0.038, and r = −0.339, p = 0.043, respectively) and the C2-C7 SVA (r = 0.648, p < 0.001, and r = 0.436, p = 0.008, respectively). The postoperative C2-C7 SVA was directly proportional to the preoperative C2-C7 SVA and postoperative C1-C2 angle (postoperative C2-C7 SVA = 0.72 + 0.669 × [preoperative C2-C7 SVA] + 0.555 × [postoperative C1-C2], r2 = 0.677, p < 0.001). The postoperative C2-C7 SVA correlated with postoperative VAS (rs = 0.382, p = 0.021) and NDI (rs = 0.476, p = 0.003).The postoperative C2-C7 SVA was affected by the preoperative C2-C7 SVA and the postoperative C1-C2 angle and showed significant positive correlation with postoperative NDI. The C1-C2 fixation angle and the preoperative C2-C7 SVA should be carefully considered to avoid postoperative sagittal imbalance.