Purpose: C1-C2 instability or painful osteoarthritis are recognised indications for posterior atlanto-axial fixation. In the traditional trans-articular C1-C2 screw fixation, up to 20% of patients cannot have safe placement of bilateral screws in the event of a medially located vertebral artery and a straight screw trajectory in the sagittal plane. The more recently developed C1-C2 fixation technique with individual C1 lateral mass screws and converging C2 pars screws can be employed in case of a medially located vertebral artery and has comparable biomechanical strength. This is a prospective observational study to investigate the advantages, the safety, and the drawbacks of posterior atlanto-axial fixation with polyaxial C1 lateral mass screws and C2 pars screws. Methods: Twelve consecutive patients with C1-2 instability (n = 11) and painful osteoarthritis (n = 1) underwent a posterior atlanto-axial fixation with polyaxial C1 lateral mass screws and C2 pars screws. The average follow-up was 16months and all patients reached the 12-month follow-up. Findings: No hardware failure occurred in any of the patients. Correct screw placement and construct stability was found in all 12 patients (100%) at 6 and 12months after surgery. Mean neck pain on a visual analogue scale (VAS) was 2.1 at 6months and 2.0 at 12months. Only transient complications were observed: one patient presented with progressive intestinal herniation through the iliac crest scar; one suffered from severe pain at the posterior iliac crest for 3 months and three patients complained of annoying pain/dysaesthesia in the C2 dermatome for 3-6months after surgery. Conclusion: This study confirms that posterior atlanto-axial fixation with polyaxial C1 lateral mass screws and C2 pars screws is a safe and effective surgical option in the treatment of atlanto-axial instability or painful osteoarthriti
