Fig. 3

Measurement of the COR at the operated level (C5–C6). a–c Aided by the alignments between P1–P5 and P1'–P5' in C6 vertebra, the flexion and extension radiographs were superimposed. c Three anatomical landmarks in C5 vertebra were located and labeled as P6–P8 in the flexion. Similarly, the corresponding markers were labeled as P6'–P8' in the extension. d COR was determined as the converging point of the three perpendicular bisectors (corresponding to segments P6P6', P7P7', and P8P8'). Referring to the procedures outlined by Amevo et al. [26], a rectangular coordinate system was set up using the tangents of C6 vertebral body. The forward and upward directions are positive directions of the X- and Y-axis, respectively. OX1 and OY1 correspond to the width and height of C6 vertebral body, respectively. The location of COR is noted as the following, COR (X, Y) = [(OX2/OX1) * 100%, (OY2/OY1) * 100%]