Fig. 1

Radiographic measurements using the Cobb’s angle. a In the neutral–lateral radiograph, the lordosis angle of the overall cervical spine (red lines) and operated level (yellow lines) was defined as the angles between the tangents of the corresponding vertebral endplates. A positive value was assigned to an angle with an anterior opening, while the negative value presented the posterior opening. b, c In the flexion–extension radiographs, the overall cervical ROM and segmental ROM (ROM of the operated level, SROM, and IROM) were the flexion–extension difference of the overall cervical lordosis angle and intervertebral angle (between the two yellow lines), respectively