Fig. 1

A, cervical X-ray in the neutral position: cSVA (a), the horizontal distance from plumbline dropped from the C2 vertebral body to the posterosuperior corner of the C7 vertebra; CL (b), the angle between the C2 lower endplate and the C7 lower endplate; B and C, cervical X-ray in the flexion and extension position: Flex CL (c) and Ext CL (d) refer to CL in flexion and extension positions. cSVA, cervical sagittal vertical axis; CL, cervical lordosis; Flex CL, cervical lordosis in flexion; Ext CL, cervical lordosis in extension