Fig. 4

Radiographic parameters. a Atlantoaxial angle (A-A angle, the C1-C2 angle). The atlantoaxial fixation angle, which is the angle formed by the line connecting the lower margins of the anterior and posterior arches of the C1 vertebra and the lower margin of the C2 vertebra. b Range of motion between the occipital bone and the atlas (ROM of O-C1). On the lateral view of the cervical spine, the range of motion of the angle formed by the line connecting the anterior and posterior margins of the foramen magnum and the line connecting the anterior and posterior lower margins of the C1 arch, i.e., the range of motion between the occipital bone and atlas, was assessed. c Atlantoaxial height (A-A height). The height of the atlantoaxial joint, corrected for the distance between the upper margin of the anterior arch of the C1 vertebra and the lower margin of the C2 vertebral body, on a lateral-view plain radiograph is shown. d Alignment of the entire cervical spine. Lordosis, straight, s-shaped, or kyphosis