Safe zone for C1 lateral mass screws: anatomic and radiological study.

Objective: To evaluate the possible complications of overpenetrated C1 lateral mass screws and to identify and define a "safe zone" area anterior to the C1 vertebra.

Methods: The study was performed on 10 cadavers and 50 random patients who had undergone computed tomographic scanning with contrast medium of the neck for other purposes. Atlas lateral mass screw trajectories were plotted, and the safe zone for screw placement anterior to the atlas vertebra was determined for each trajectory.

Results: The trajectory of the internal carotid artery was measured from its medial wall. The trajectory of the internal carotid artery according to the ideal entrance point of the screw was 11.55 +/- 4.55 degrees (range, 2-22 degrees) in the cadavers and 9.78 +/- 4.55 degrees (range, -5 to 22 degrees) bilaterally in the patients. At 15 degrees (ideal screw trajectory), the thickness of the rectus capitis anterior muscle and longus capitis muscle was 6.69 +/- 0.83 mm (range, 5.32-7.92 mm) in the cadavers and 7.29 +/- 1.90 mm (range, 0.50-13.63 mm) bilaterally in the patients. The smallest distance from the internal carotid artery to the anterior cortex of the C1 vertebra was calculated as 4.33 +/- 2.03 mm (range, 1.15-8.40 mm) bilaterally in the cadavers and 5.07 +/- 1.72 mm (range, 2.15-8.91 mm) bilaterally in radiological specimens.

Conclusion: The internal carotid artery trajectory is lateral to the ideal entrance point of C1 lateral mass screws. The medial angulation of a screw placed in the lateral mass of C1 seemed to increase the margin of safety for the internal carotid artery. The rectus capitis anterior and longus capitis muscles may be thought of as a safe zone area for C1 lateral mass screws. At more than 25 degrees of medial angulation, the risk of perforation of the oropharyngeal wall increases.