(a) Pelvic model showing the oblique plane of the axial CT scan cuts which is used for preoperative calculation of safe screw angles and the anatomical plane perpendicular to the acetabulum. The latter is the usual plane of screw placement. (b) Diagrammatic illustration showing the posterior wall as a three-sided prism with the two planes described in (a). Due to the obliquity of the CT plane, and are longer than and . This makes angles 3 and 4 in the oblique plane bigger than angles 1 and 2 in the perpendicular plane. Since equals angle which represents the retroacetabular angle in CT cuts and is used in calculation of safe screw angles is smaller than which represents the retroacetabular angle measured in a plane perpendicular to the acetabulum. The retroacetabular angle measured on CT is thus a safer angle.