Fig. 1. Lateral radiographs and diagrams in extension of the entire cervical spine (  A ) and the sniffing position (  B ) showing marked atlantoaxial subluxation of the anterior atlas–dens interval (AADI) of 10 mm and the posterior atlas–dens interval (PADI) of 12 mm in the sniffing position, compared with an AADI of 2 mm in extension of the entire cervical spine. The AADI and PADI are defined as the distance between the posterior surface of the anterior arch of the atlas and the anterior surface of the dens and the distance between the posterior surface of the dens and the anterior surface of the posterior arch of the atlas, respectively.  7,In addition, the subluxation occurs at the C4–C5 level in the sniffing position (  B ). Radiographs are taken when the patient lies down on the flat table, and the head and neck position is determined by actively positioning with the patient’s effort. The sniffing position is achieved by placement of 7 cm of a pillow under the head,  8,9,and extension of the entire cervical spine is accomplished by bending part of the table. Despite the same degree of occipitoatlantoaxial extension, severe subluxation occurs only in the sniffing position. Reference lines for the occiput (C0) and the axis (C2) are defined as the McGregor line and the line passing through anterior and posterior basal plate of the C2 vertebral body, respectively.  14