C1 anatomy and dimensions relative to pedicle screw placement

The recent C1 pedicle screw technique for upper cervical vertebral stabilization allows longer screws to be implanted by setting the screw entry point through the posterior arch of C1, which could provide better biomechanics. However, there is controversy regarding the placement of C1 pedicle screws at different angles. We retrospectively reviewed the computed tomography (CT) scans of 300 patients. The trajectories of medial inclination of 0°, 5°, 10°, and 15° and trajectory of the maximum medial inclination angle were designed for each C1 pedicle on CT images. Screw track length at each angle, the angle of maximum medial inclination, pedicle height, distance from the screw entry point to the midpoint of the C1 posterior tubercle, and screw perforation rate at each angle were measured. The average maximum inclination angle was 17.01°, the maximum inclination angle screw track length was 31.05 mm, and the distance from the screw entry point to the midpoint of the C1 posterior nodule was 21.65 mm. The screw perforation rate was 46.73% at 15° of medial inclination, but only 5.61% at 10°, and no screw perforation at 5°. 26.47% C1 pedicle height < 4 mm. There was no significant difference between the measured data on the left and right sides(P > 0.05), and the measurement of female patients was usually smaller and significantly different from that of male patients(p < 0.05). Our data indicate that a reasonable screw inclination angle of 10° and the safety zone of screw angle can provide safety and avoid screw perforation. However, personalized measurement before surgery is essential.