The ilium (also iliac bone, Latin: os ilium) is a paired bone of the pelvic skeleton and the uppermost and largest part of the hip bone.


Each iliac bone has two parts, which are:

  • body of the ilium, and
  • wing of the ilium.

The body of the ilium is the central portion of the ilium situated near the acetabulum. It contributes to forming less than two-fifths of the acetabulum, also forming part of the acetabular fossa. The internal surface of the body of the ilium forms part of the lesser pelvis and serves as the origin site of some fibers of the obturator internus muscle.

The wing of the ilium is the large expanded portion of the ilium. The wings of the ilium represent the lateral borders of the greater pelvis.


The ilium also presents several important landmarks, such as:

  • arcuate line,
  • iliac crest, featuring
    • anterior superior iliac spine,
    • anterior inferior iliac spine,
  • iliac fossa,
  • sacropelvic surface, presenting
    • auricular surface,
    • iliac tuberosity,
  • gluteal surface, featuring
    • anterior gluteal line,
    • posterior gluteal line,
    • inferior gluteal line.

The arcuate line is a prominent bony ridge on the internal surface of the ilium separating the body and the wing of the ilium, also forming the boundary between the greater and lesser pelvis.

The iliac crest is the superior border of the wing of the ilium, also forming the superolateral margin of the greater pelvis. Two bony projections mark the iliac crest. The anterior superior iliac spine marks the anterior limit of the iliac crest and serves as the origin site for the sartorius muscle. The anterior inferior iliac spine is a bony process on the anterior margin of the ilium. The rectus femoris muscle originates here.

The iliac fossa is a concavity forming the internal surface of the wing of the ilium.

The sacropelvic surface of the ilium is the side of the dorsal segment of the ilium that faces the sacrum. It consists of two parts: the auricular surface and the iliac tuberosity. The auricular surface is an ear-shaped surface within the sacropelvic surface of the ilium, which articulates with the sacrum. The auricular surface is covered with fibrocartilage. The iliac tuberosity is a roughened area behind and above the auricular surface. It serves as the attachment site for the sacroiliac ligaments.

The gluteal surface of the ilium is the external surface of the wing of the ilium, which presents with three ridges: the anterior, posterior, and inferior gluteal lines. The anterior gluteal line is a flat ridge situated almost in the middle of the wing of the ilium on its external surface between the sites of origin of the gluteus medius and gluteus minimus muscles. The posterior gluteal line is a bony ridge on the outer surface of the wing of the ilium between the fields of origin of gluteus medius and gluteus maximus muscles. The inferior gluteal line is a ridge located above the acetabulum between the sites of origin of the gluteus minimus and rectus femoris muscles.