Each innominate consists of three bones: the ilium, ischium, and pubis. These bones are fused together in adults and make up the left and right side of the pelvis.
The acetabulum is formed at the union of all three bones. The acetabulum is a cup-shaped articular feature that mates with the head of the femur forming the ball-and-socket joint of the hip. The concave acetabulum is deeper than other ball-and-socket joints, adding to the geometric stability of the hip joint.
The ilium is the largest part of the innominate. Like other flat bones, it acts as a shield to protect internal organs and as an anchor for muscles. The ilium has several important anatomical landmarks including the iliac crest and the anterior and posterior superior iliac spines. The lower end of the ilium is fused with the ischium and the pubis at the acetabulum.
The ilium provides an attachment point for several muscles important for locomotion and upper body movement, including the latissimus dorsi, external oblique, transverses abdominis, quadratus lumborum, gluteus maximus, erector spinae, iliacus, and rectus femoris.
The ischium makes up the lower posterior portion of the hip bone. A major feature is the ischial tuberosity, which supports body weight in the sitting position. The ischium also provides an attachment point for several muscles, including the quadratus femoris, biceps femoris, and the adductor magnus.
The pubis makes up the anterior portion of the hip bone. It provides an attachment point for the thigh adductor muscles and articulates with the opposite pubis at the pubic symphysis.
