Fig. 4
From: Digital anatomical study and clinical application of the ideal S2 alar-lliac screw trajectory
HD, the horizontal distance from the point of entry to the median sacral spine (A); VD, the vertical distance from the point of entry to the inferior margin of the first posterior sacral foramen; SA, the angle between the projection of the screw trajectory in the sagittal plane and the horizontal line (B); TA, the angle between the projection of the screw trajectory in the transverse section and the median sagittal line (C)