A) Anatomical regions of the internal aspect of the innominate bone are illustrated. The quadrilateral surface is bound by the pelvic brim anteriorly, the greater and lesser sciatic notches posteriorly, the obturator foramen inferiorly, and the sciatic buttress superiorly. The iliopectineal eminence lies directly over the anterior wall of the acetabulum. The nutrient foramen of the iliac wing is a consistent landmark found adjacent to the sacroiliac joint. B) The external aspect of the innominate bone is shown. The retroacetabular surface is outlined.