To avoid neurovascular injuries and to establish the ideal trajectory of screw insertion, a computed tomographical evaluation was conducted.

Twenty volunteers without cervical disease were employed for this study. Axial and oblique CT scans were selected for evaluation. Oblique CT scans were obtained in the direction of provisional screw insertion, starting from the junction of the lamina and the inferior articular facet of the axis and extending to the center of anterior tubercle of the atlas. In the axial and oblique reconstructed views, the screw length, the diameters of the medial and lateral cortexes of the isthmus, the vertical distance, and the angle of C1 were measured.

The screw lengths averaged 37.6 mm (35.1-40.5 mm) in males and 37.2 mm (33.6-43.4 mm) in females. The diameters of the medial cortex of the isthmus were 4.3 mm (3.3-5.2 mm) in males and 4.0 mm (2.8-5.6 mm) in females. No measurement was statistically different between males and females.

Due to individual variations of atlantoaxial anatomy, especially in terms of the size of cortical diameter of the isthmus, 3-D reconstruction CT is a useful tool for planning transarticular C1-2 screw insertion.