Acetabular quadrant system described by Wasielewski et al. for determining safe screw placement (see text). Quadrants are formed by intersections of lines A and B. Line A extends from anterior superior iliac spine (ASIS) through center of acetabulum to posterior aspect of fovea, dividing acetabulum in half. Line B is drawn perpendicular to line A at midpoint of acetabulum, dividing it into quadrants: anterosuperior, anteroinferior, posterosuperior, and posteroinferior.