Ilium

The ilium is the largest and uppermost of the three bones that form each hip bone, or os coxae. The other two are the ischium and pubis. In adults, these three bones are fused, but their anatomical regions remain identifiable. The ilium forms the broad upper part of the hip bone and contributes substantially to the structure of the pelvis .¹

The ilium consists mainly of an expanded, wing-shaped portion called the ala and a thicker inferior portion called the body. The upper border of the ala forms the iliac crest, which extends between the anterior superior and posterior superior iliac spines. On the inner surface of the ala is a broad concavity called the iliac fossa .¹

Posteriorly, the ilium has an ear-shaped auricular surface that articulates with the sacrum to form the sacroiliac joint. Inferiorly, the ilium contributes to the acetabulum, the socket that receives the head of the femur at the hip joint. Through these relationships, the ilium helps form the structural connection between the axial skeleton and the lower limb .¹

The ilium should not be confused with the entire hip bone. The ilium is its superior component, whereas the mature hip bone consists of the fused ilium, ischium, and pubis .¹