Radiographic measurement of the C1 lateral mass for posterior atlantoaxial fixation in patients with a persistent first intersegmental artery.

Background: Posterior C1-2 fixation with C1 lateral mass and C2 pedicle screws is an established treatment for atlantoaxial instability. In patients with a persistent first intersegmental artery (PFIA), the vertebral artery courses beneath the C1 posterior arch and may restrict safe C1 screw placement. This study quantified C1 lateral mass morphology in PFIA and assessed the radiographic feasibility of 3.5-mm screw placement. Methods: Cervical computed tomography angiograms or high-resolution computed tomography scans of 346 adults were retrospectively reviewed. Thirteen patients with PFIA, involving 14 sides, were compared with 26 age- and sex-matched controls. Parasagittal reconstructed images were used to measure C1 lateral mass and posterior arch dimensions. Vertical clearance for a 3.5-mm screw was assessed above the intact posterior arch and after simulated partial posterior arch notching. Results: PFIA was identified in 13 of 346 patients (3.8%). Overall, C1 lateral mass height and posterior arch thickness were preserved, whereas the inferior lateral mass was larger on the PFIA sides. Distance E was <3.5 mm in 10 of 14 PFIA sides (71%), whereas Distance F was ≥3.5 mm in 13 of 14 sides (93%). Conclusion: In PFIA, the intact posterior arch frequently limits the superior-entry C1 screw corridor. Partial posterior arch notching expanded the available corridor to ≥3.5 mm in approximately 90% of sides. Preoperative computed tomography angiography and individualized fixation planning are essential.