The acetabulum's a surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint. Structure There're three bones of the os coxae (hip bone) that come together to form the acetabulum. Contributing a little more than two-fifths of the structure's the ischium, which provides lower and side boundaries to the acetabulum. The ilium forms the upper boundary, providing a little less than two-fifths of the structure of the acetabulum. The rest's formed by th… (More on Acetabulum)