Sagittal cervical alignment parameters. (a–b) Oc–C1 angle, (b–c) C1–2 angle, (d–e) C2–7 angles, and (f) C2–7 sagittal vertical axis (SVA) were measured between the lines shown in the figure. The Oc–C1 angle was calculated by subtracting the angle between the McGregor line and the line passing through the inferior aspect of the atlas in extension from that in the forward flexion. The C2–7 SVA was defined as the distance from the posterosuperior corner of C7 and the vertical line from the center of the C2 body. The C2–7 angle was calculated by subtracting the angle between the line drawn horizontal to the inferior endplate of the C–2 vertebral body and the line drawn horizontal to the inferior endplate of the C–7 vertebral body in extension from that in the forward flexion.