Fig. 1.
Lateral fluoroscopy images demonstrating the methods to measure anterior-posterior spinal distances. (a) C2 inferior endplate length was measured as distance between the anterior and posterior surfaces of C2 at the inferior border of C2. (b) C2 canal space was measured as shortest distance between the posterior surface of the C2 vertebral body and the midpoint of C2 spinolaminar line. (c) C1-C2 canal space was defined as the smallest anterior-posterior distance available to the cord at this level, measured in one of two ways. As shown in A, when the C1-odontoid complex was anteriorly displaced, C1-C2 canal space (c) was measured between the posterior surface of the C2 vertebral body and the spinolaminar line between C1-C2. As shown in B, when the C1-odontoid complex was posteriorly displaced, C1-C2 canal space (c) was measured between the posterior-inferior surface of the odontoid and the spinolaminar line between C1-C2.