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Portrait of Ilium

Anatomy · Bones

Ilium

The largest and uppermost part of the hip bone

Pelvic girdle · hip bone

The ilium is the large, fan-shaped upper part of the hip bone, forming the prominence of the hip. Its curved upper margin, the iliac crest, is what you feel with your hands on your waist. It joins the sacrum behind at the sacroiliac joint and, with the ischium and pubis, forms the acetabulum.

How to spot them

  • ▸Broad, fan-like blade forming the upper part of the hip bone, with a curved upper border (the iliac crest)
  • ▸Crest ends in the anterior superior iliac spine in front and the posterior superior iliac spine behind
  • ▸Roughened, ear-shaped auricular surface on the inner face, meeting the sacrum
  • ▸A shallow iliac fossa on the inner surface, bounded below by the arcuate line, and a greater sciatic notch behind

What to know

The uppermost, fan-shaped and largest part of the hip bone, whose curved upper margin can be felt with the hands on the waist

Ilium — with the iliac crest.

A line joining the highest points of its upper margins crosses the spine at about the fourth lumbar vertebra, and guides lumbar puncture

Ilium — Tuffier's (intercristal) line.

Its ear-shaped surface unites it to the sacrum at a strong, almost immobile joint

Ilium — the auricular surface and the sacroiliac joint.

Bony points at the back of its upper margin lie beneath the 'dimples of Venus' in the small of the back

Ilium — the posterior superior iliac spines.

Bone marrow is often aspirated from its back, and bone grafts are taken from its front

Ilium — the posterior and anterior iliac crest.

A broad, shallow concavity on its inner surface is bounded below by a ridge forming part of the brim of the true pelvis

Ilium — the iliac fossa and the arcuate line.

It contributes the upper two-fifths of the cup that receives the head of the thigh bone

Ilium — about two-fifths of the acetabulum.

Key ideas

  • The iliac crest is a landmark for many procedures: lumbar puncture (Tuffier's line at L4), bone marrow biopsy and bone graft harvest.
  • The anterior superior iliac spine and the pubic tubercles lie in the same vertical plane in the anatomical position, which describes the tilt of the pelvis.

Quick-fire facts

  • •It is the largest of the three bones that make up the hip bone (ilium, ischium, pubis).
  • •The iliac crest is the curved upper margin; the anterior superior iliac spine (ASIS) at its front is palpable at the front of the hip.
  • •Posteriorly the crest ends at the posterior superior iliac spine, above the 'dimples of Venus' (sacral dimples).
  • •The auricular surface articulates with the sacrum to form the sacroiliac joint.
  • •The arcuate line of the ilium forms part of the pelvic brim.
  • •The greater sciatic notch is the large indentation in the posterior margin of the lower ilium.
  • •Tuffier's line, joining the tops of the iliac crests, crosses L4 (or the L4–L5 interspace).
  • •The word ilium probably comes from Latin ilia, the flanks.

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Sources