Radiological measurement of the sagittal X-ray. The regional Cobb angle is the angle between the red tangential line to the cephalad endplate line of the upper instrumented vertebrae (UIV) (B) and the red tangential line to the caudal endplate line of the lower instrumented vertebrae (D). The proximal junctional Cobb angle is the angle between the blue tangential line to the cephalad endplate line of the two supraadjacent vertebrae above the UIV (A) and the blue tangential line to the caudal endplate line of the UIV (C).