Fig. 1.  Still images (A ) before, (B ) during, and (C ) after intubation with combination of the Airway Scope (HOYA-Pentax, Tokyo, Japan) and the bougie. To minimize movement of the cervical spine during intubation attempt, the glottic exposure is limited to that necessary to allow passage of the bougie and the endotracheal tube (B ). Reference lines  for the (C0 ) occiput and the (C2 ) axis are defined as the McGregor line and the line passing through the anterior, inferior margin of the C2 vertebral body and the lower cortical margin of the spinous process, respectively.8,9,11,12,Reference lines  do not intersect on the radiograph; therefore, the C0 or C2 angle is defined as the difference in angle between the C0 or C2 reference line and the common line that is the ventral vertical edge, respectively. The angle between the C0 and C2 (C0-2 angle) in each radiograph is calculated as the difference between the C0 and the C2 angles. Extension angle of the C0-2 created by laryngoscopy is defined as the difference between the C0-2 angle before laryngoscopy and that during laryngoscopy. Laryngoscopy produces extension of 15.0 degrees at the C0-2, leading to exacerbation of atlantoaxial subluxation (B ). Note that the atlas-dens interval, which is defined as the distance between the posterior surface of the anterior arch of the atlas and the anterior surface of the dens,1,8increases from 3.7 mm (A ) before laryngoscopy to 6.8 mm (B ) during laryngoscopy and intubation. (C ) The subluxation is corrected after intubation (3.9 mm).