• Satona Murakami, Jun Mizutani, Muneyoshi Fukuoka, Kenji Kato, Isato Sekiya, Hideki Okamoto, Kuniyoshi Abumi, and Takanobu Otsuka.
• Department of Orthopaedic Surgery, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan.
• Spine. 2008 Nov 15; 33 (24): 2581-5.

Study DesignEvaluation of diagnostic imaging.ObjectiveTo comprehend anatomic relationships between the internal carotid artery (ICA) and bicortical purchase of C1 lateral mass screws from the perspective of avoiding ICA injury.Summary Of Background DataNo studies have evaluated safety trajectory of atlantal lateral mass screw that would avoid the ICA injury in relation to its location, although previous studies have indicated concern about ICA injury by the screw tip at the anterior surface of the lateral mass of the atlas.MethodsFrom 149 of 177 human 3-dimensional computed tomography reconstruction images, 6 distance and 2 angle parameters related to both atlas and ICA were measured on the plane 15 degrees cephalad to the transverse plane. In addition, angle of error during screw insertion from intended trajectory was checked.ResultsThe ICA was located in front of the C1 lateral mass in 64.4% of cases and faced the lateral one third of the C1 lateral mass in 54.6% of cases. None were located in front of the medial one third of the C1 lateral mass. The maximum inward screw trajectory that would violate the ICA was 8.6 degrees . Mean angle of preoperative C1/2 rotation and angle of error from intended trajectory was about 5 degrees , respectively.ConclusionThe possibility of ICA injury can be excluded by correct insertion of the screw 10 degrees inward. Although bicortical purchase with adequately medially angulated trajectory might be safe enough, we must remember the possibility to violate the ICA in bicortical purchase, because the intended screw trajectory never be assured.

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