• Jin S Yeom, Jacob M Buchowski, Kun-Woo Park, Bong-Soon Chang, Choon-Ki Lee, and K Daniel Riew.
• Department of Orthopaedic Surgery, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Gumi-dong, Bundang-gu, Sungnam, Kyungki, Republic of Korea.
• Spine J. 2009 Jul 1;9(7):574-9.

Background ContextInadvertent perforation of the C0-C1 and C1-C2 joints is one of the potential complications of C1 screw insertion.PurposeTo 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 DesignScrew insertion simulation using computed tomography (CT).Patient SampleCervical spine 1.0-mm interval CT scans of 154 patients performed at a single institution between October 2004 and October 2005 were analyzed.Outcome MeasuresC0-C1 and C1-C2 joint violations during CT-based simulation of C1 lateral mass screw placement.MethodsFine cut CT scans and screw trajectory software was used to simulate insertion of 4.0mm 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.ResultsScrew trajectories in 154 patients (308 screws) were evaluated. Using the 15 degrees 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 degrees angulation, safety margin was slightly wider. Because it may be difficult to differentiate between 0 degrees and 15 degrees 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."ConclusionsWhen the screw is directed between 0 degrees and 15 degrees 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.

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