Fig. 3

Lateral cervical radiograph. C1- C7 show the line from posterior border of spinal aperture to posterior tubercle of atlas or tip of spinal process; A: the base of the posterior border of the mastoid process; B: The external occipital protuberance; D: the tip of the posterior tubercle of the atlas; E: the anterior end of the inferior border of the spinous process of the axis; F: the tip of the spinous process of the axis; a: the upper border length from the tip of the posterior tubercle of the atlas to the posterior wall of the vertebral foramen; b: the lower border length from the tip of the posterior tubercle of the atlas to the posterior wall of the vertebral foramen; x: the length of the superior border of the spinous process; y: the length of the inferior border of the spinous process