Cervical sagittal parameters. (1) Cervical curvature, Jackson physiological stress curve: two lines are drawn parallel to the posterior edge of C2 and C7; the angle between the two represents the curvature of the cervical spine (a). (2) C2-7 Cobb's angle, the angle between C2 and C7 lower end plate tangent (b). (3) C2–C7 sagittal vertical axis (SVA), the horizontal distance between the back angel of C7 upper end plate and the vertical line of the geometric center of C2 vertebral body (d).