Fig. 1

A Illustration of the C2 subfacetal screw for KFS. The selected entry point was approximately 3 to 4 mm below the midpoint of the posterior edge of the superior articular surface and parallel to the superior articular surface or slightly downward. The trajectory should be tilted inward as far as possible to obtain a longer stitching track. B Measurements of the internal height and isthmus height of patients with high-riding vertebral arteries (HRVAs). The isthmus height decreased significantly. C Illustration of the C2 subfacetal screw. The internal height provides sufficient space for the insertion of the C2 subfacetal screw