Background context: Inadvertent perforation of the C0-C1 and C1-C2 joints is one of the potential complications of C1 screw insertion. Purpose: To identify a simple lateral fluoroscopic landmark to help prevent atlantooccipital (C0-C1) and atlantoaxial (C1-C2) joint violations during C1 lateral mass screw insertion. Study design: Screw insertion simulation using computed tomography (CT). Patient sample: Cervical spine 1.0-mm interval CT scans of 154 patients performed at a single institution between October 2004 and October 2005 were analyzed. Outcome measures: C0-C1 and C1-C2 joint violations during CT-based simulation of C1 lateral mass screw placement. Methods: Fine cut CT scans and screw trajectory software was used to simulate insertion of 4.0 mm screws. The entry point was the middle of the junction of the posterior arch and the posterior inferior part of the lateral mass. Zero and fifteen degrees medially angulated trajectories were evaluated. For both, we determined the maximum cranial and caudal angulation that avoided joint violation, and where the screw could safely be directed in the C1 anterior arch on a lateral view using these angulations. We expressed these targeting points as a percentage of the total height of the anterior atlas arch such that 100% represented the cranial border of the arch, 50% the center and 0% the caudal border. Results: Screw trajectories in 154 patients (308 screws) were evaluated. Using the 15° medial angulation, the C0-C1 joint was safe in all cases when the trajectory was below the 40% point of the anterior arch. The C1-C2 joint was safe when the trajectory was above the 20% point. Using the 0° angulation, safety margin was slightly wider. Because it may be difficult to differentiate between 0° and 15° of medial angulation intraoperatively, we suggest aiming the screw tip between the 20% and 40% points for either trajectory. We call this the "safe zone of C1.". Conclusions: When the screw is directed between 0° and 15° medially, it can be inserted without C0-C1 and C1-C2 joint violation if the screw tip trajectory lies between the 20% and 40% points of the anterior atlas arch.
- Atlantoaxial joint violation
- Atlantooccipital joint violation
- C1 lateral mass screw
- CT scan
- Screw insertion