Fig. 1

Point (a) in the figure shows the screw entry region, which is 4 mm above the junction between the inferior edge of the lateral arch of the axis and the lateral edge of the vertebral body of the axis. Point (b) shows the implantation direction of the device and (b) is the midpoint of the posterior edge of the superior articular process of the atlas. Point (c) indicates the screw path, and point (d) is the 5 mm safety buffer zone