Although C1-C2 transarticular screw fixation has become a popular surgical method of treating atlantoaxial instability, we remain concerned about the potential for injury to the vertebral artery. Because of the lack of an objective measuring method, surgical unsuitability has been decided on the basis of individual experiences as reported in 18-23% of cases. In this study, the point of screw intersection(the passing points) on the superior articular surface of C2 were measured and the directions of these were thus objectified. Sixty-four healthy volunteers underwent 1mm fine-slice C1-C2 CT scanning, and sagittal images were reconstructed at 3.5mm(3.5mm lateral image) and 6mm(6mm lateral image) lateral to the spinal canal. The C1/2 transarticular screw trajectories making the longest paths or violating the transverse foramen(dangerous trajectory) were drawn and the distance from their points of screw intersection on the superior articular surface of C2 were measured from the posterior rim of the superior articular surface of C2. When the space available for screw(SAS) posterior to the passing point of the dangerous trajectory was equal to or less than 3.5mm(major diameter of the commonly used screw), the case was considered unacceptable, when SAS was over 3.5mm but equal to or less than 4.5mm, screw placement were considered risky. The trajectories made the longest paths when they passed 3.6+/-1.6mm(mean+/-S.D.) and 2.8+/-1.7mm(mean+/-S.D.) anterior to the posterior rim of the posterior articular surface of C2 as seen on 3.5mm lateral images and 6mm lateral images, respectively. While four of 64 cases(6.3%) were unilaterally unacceptable or risky on 3.5mm lateral images, 21 cases(32.8%) were unacceptable or risky on 6mm lateral images(unilateral, 15cases; bilateral, 6 cases). When the trajectories inclined forward to 0%, 25%, 50%, 75% and 100% points of AP diameter of the superior articular surface of C2 from the posterior rim, the respective risks of the involvement of the transverse foramen increased to 0.78%, 1.5%, 25%, 74% and 100%, as seen on 3.5mm lateral images and 10.9%, 14%, 62.5%, 95% and 100%, on 6mm lateral images.