Influence of Atlantoaxial Fusion on Sagittal Alignment of the Occipitocervical and Subaxial Spines in Os Odontoideum with Atlantoaxial Instability

Study Design: Retrospective case analysis. Purpose: We hypothesized that larger the C1-C2 fusion angle, greater the severity of the sagittal malalignment of C0-C1 and C2- C7. Overview of Literature: In our experience, instances of sagittal malalignment occur at C0-C1 and C2-C7 following atlantoaxial fusion in patients with Os odontoideum (00). Methods: We assessed 21 patients who achieved solid atlantoaxial fusion for reducible atlantoaxial instability secondary to 00. The mean patient age at the time of the operation was 42.8 years, and the mean follow-up duration was 4.9 years. Radiographic parameters were preoperatively measured and at the final follow-up. The patients were divided into two groups (A and B) depending on the C1-C2 fusion angle. In group A (n=11), the C1-C2 fusion angle was 22 degrees, whereas in group B, it was 22 degrees. The differences in the radiographic parameters of the two groups were evaluated. Results: At the final follow-up, the C1-C2 angle was increased. However, this increase was not statistically significant (18 degrees vs. 22 degrees, p=0.924). The C0-C1 angle (10 degrees vs. 5 degrees, p<0.05) and C2-C7 angle (22 degrees vs. 13 degrees, p<0.05) significantly decreased. The final C1-C2 angle was negatively correlated with the final C0-C1 and C2-C7 angles. The final C0-C1 angle (4 degrees vs. 6 degrees, p<0.05) and C2-C7 angle (8 degrees vs. 20 degrees, p<0.05) were smaller in group A than in group B. After atlantoaxial fusion, the C0-C1 range of motion (ROM; 17 degrees vs. 9 degrees, p<0.05) and the C2-C7 ROM (39 degrees vs. 31 degrees, p<0.05) were significantly decreased. Conclusions: We found a negative association between the sagittal alignment of C0-C1 and C2-C7 after atlantoaxial fusion and the C1-C2 fusion angle along with decreased ROM. Therefore, overcorrection of C1-C2 kyphosis should be avoided to maintain good physiologic cervical sagittal alignment.