Posterior Atlantoaxial Fixation in a Single-Center Consecutive Series: Anatomy-Driven C2 Anchorage Selection

AIM: To present a single-center experience with posterior C1-C2 fixation for atlantoaxial instability, emphasizing an anatomy-driven approach to C2 anchorage selection, construct strategy, complications, and clinical outcomes. MATERIAL and METHODS: We retrospectively reviewed 45 patients who underwent posterior C1-C2 fixation between June 2020 and January 2026. Constructs were categorized as pedicle, hybrid, bilateral laminar, planned C2 omission (C1-C3 extension), or transarticular fixation. Demographics, etiology, operative variables, complications, reoperations, mechanical failure, and fusion were analyzed. Exploratory categorical comparisons were performed using Fisher's exact test. Fusion was evaluated using postoperative computed tomography (CT) obtained ≥6 months postoperatively and was defined as osseous bridging without implant loosening. RESULTS: The mean patient age was 47.8 ± 20.1 years, and 25 patients (55.6%) were women. Etiologies included trauma (31.1%), basilar invagination (22.2%), Chiari malformation (15.6%), os odontoideum (8.9%), atlantoaxial subluxation (8.9%), tumor (8.9%), and other causes (4.4%). Construct types included pedicle fixation in 31 patients (68.9%), hybrid constructs in 5 (11.1%), bilateral laminar fixation in 5 (11.1%), planned C2 omission in 3 (6.7%), and transarticular fixation in 1 patient (2.2%). The median follow-up duration was 6.3 months (IQR, 1.2-22.3 months). CT-based fusion assessment was available for 26 patients at a median follow- up of 19.5 months, and fusion was observed in 20 patients (76.9%). Radiographic mechanical abnormalities were identified in 7 patients (15.6%), including 6 (13.3%) who underwent revision surgery. Construct strategy was not significantly associated with either mechanical abnormalities or CT-confirmed fusion. Mechanical abnormalities were significantly more frequent in patients with os odontoideum or nontraumatic atlantoaxial subluxation than in those with other etiologies (p < 0.001). CONCLUSION: An anatomy-driven escalation strategy for C2 anchorage selection provided a practical framework for managing variable craniovertebral anatomy during freehand posterior atlantoaxial fixation. Hybrid constructs, bilateral laminar fixation, and planned C2 omission served as feasible alternatives when pedicle fixation was limited by anatomical constraints or pathology.