The acetabular index is the angle formed between Hilgenreiner line (a horizontal line through the upper margin of the radiolucent triradiate cartilage) and a tangential line to the lateral ossific margin of the roof of the acetabulum. The acetabular index changes with axial and sagittal plane rotation of the pelvis and is helpful in measuring the development of the osseous roof of the acetabulum. In the newborn, the acetabular index must be over 40° to be significantly abnormal.
