Fig. 1
The measurement of O-C2A, C2–7A(a) and PSTT(b) on postoperative lateral X-rays. The angle between the inferior margin of the C2 vertebrae and the inferior margin of the C7 vertebrae was measured as the C2–7 angle while the angle between the McGregor’s line and the inferior margin of the C2 vertebrae was measured as the O-C2 angle perioperatively on plain lateral cervical radiographs (a). The PSTT was measured at the extended line of the AP diameter from C2 to C7, and the AP diameter was measured between the center of the posterior and anterior cortex of each vertebral body. O-C2A: O-C2 angle, C2–7A: C2–7 angle, AP: Anteroposterior, PSTT: Prevertebral soft tissue thickness