Fig. 2

Measurement of AADI Method and the Swischuk Line Method. The AADI method is defined as the measurement of the distance between the posteroinferior aspect of the anterior arch of C1 and the most anterior aspect of the odontoid process (depicted by a black line). The Swischuk line method is defined as the measurement of the distance (depicted by a yellow dashed arrow) from a line drawn from the anterior aspect of the posterior arch of C1 to the anterior aspect of the spinolaminar junction of C3 (Swischuk line, depicted by a yellow line) to the spinolaminar junction of C2 (yellow dashed line). If the spinolaminar junction of C2 is located dorsal to the Swischuk line, as in this case, it is documented as positive values, whereas if the spinolaminar junction of C2 is located ventral to the Swischuk line, it is documented as a negative value