ABSTRACT
STUDY DESIGN: Surgical simulation using CT images and computer software. OBJECTIVES: We wanted to determine a new trajectory for the C2 screw, and then we wanted to evaluate its safety and accuracy. Summary of Literature REVIEW: There have been a few suggestions for trajectories of the C2 pedicle screws. However, their safety is somewhat unsatisfactory as all of them have some possibility of vertebral artery injury. MATERIALS AND METHODS: Using 1 mm-sliced CT scan images of 158 patients and an A-view spine surgery simulator 1.0, we determined a new trajectory for the C2 screw with which 4.0 mm screws can be inserted with a minimal number of breaches of the bone cortices. The percentage of cortical perforations by the suggested trajectory was compared with that by the pedicle screws by means of simulation. RESULTS: The medial angulation of the determined trajectory is the same as that of the pedicle on the axial CT images: it usually is between 30 and 40 degrees. The screw is angled toward the antero-superior end of the superior articular process of C2, as observed on lateral fluoroscopy. The entry point is 3 mm inferior to the posterior aspect of the superior articular surface, and this point should allows the screw (ED note: check this and it wasn't clear.)the screw to be inserted close to the superomedial border of the superior articular process. Using this trajectory, 2.5% (8/316) of the screws breached the vertebral grooves, while 6.0% (19/316) of the pedicle screws breached them (p=0.030, chi-square test). CONCLUSIONS: The subarticular screw has improved safety compared with the pedicle screws. However, there are still some casesin which screws can not be inserted without breaching the vertebral groove. Therefore, preoperative thin-slice CT scanning with three-dimensional reconstruction and/or three-dimensional CT-angiography is recommended.