Anatomic study for ideal and safe posterior C1-C2 transarticular screw fixation

Study Design. Directions of the C1-C2 posterior transarticular screw trajectories making the longest path or violating the transverse foramen were measured by using an objective measuring method. Objectives. To clarify the directions of the screw trajectory marking the longest paths without violating the transverse foramen. To achieve this, diverse directions of the screw trajectories were objectified by measuring the locations of the points of screw intersection on the superior articular surface of C2. Summary of Background Data. The principal limitation of posterior C1-C2 posterior transarticular screw fixation is the location of 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% to 23% of cases. Methods. Sagittal reconstructed computed tomographic images were made at 3.5 mm and 6 mm from the spinal canal. C2-C2 transarticular screw trajectories making the longest path or violating the transverse foramen (dangerous trajectory) were drawn, and 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 I: available for the screw behind the points of screw intersection by the dangerous trajectory was equal to or less than 3.5 mm, the case was defined as "unacceptable"; wen the space available for the screw was more than 3.5 mm but equal to or less than 4.5 mm, it was defined as "risky" for the placement of the screw. Results. Trajectories make the longest paths when they pass an average of 3.6 mm and 2.8 mm anterior to the posterior rim of the posterior articular surface of C2 at 3.5-mm lateral images and 6-mm lateral images, respectively. Four of 64 cases were unacceptable or risky unilaterally on 3.5-mm lateral images, and 21 cases were unacceptable or risky on 6-mm lateral images. A sigmoid-shaped increment curve of the risk was noted as the increasing forward inclination of the screw trajectories increased. Conclusions. The areas on the superior articular surface of C2 intersected by the trajectories making the longest paths without violating the transverse foramen are clarified as a guide to the ideal and safe trajectories, The theoretical minimal risk and usual risk of the posterior C1-C2 transarticular screw fixation are presented as well.