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Portrait of Sacroiliac joint

Anatomy · Joints & cartilage

Sacroiliac joint

Links spine to pelvis

Pelvis · sacrum and ilium

The sacroiliac joint connects the auricular (ear-shaped) surfaces of the sacrum and the ilium on each side of the pelvis. It is a largely immobile joint, held by some of the strongest ligaments in the body, and it transmits the weight of the upper body from the spine to the pelvis and legs. It loosens slightly in pregnancy, and inflammation here is often the first sign of ankylosing spondylitis.

How to spot them

  • ▸Where the sacrum wedges between the two iliac bones, beside the posterior superior iliac spines
  • ▸Ear-shaped (auricular) surfaces with irregular ridges and grooves that interlock
  • ▸Synovial in its lower front part, ligamentous (a syndesmosis) behind
  • ▸Surface dimples ('dimples of Venus') lie over the posterior superior iliac spines, near the joints

What to know

A joint where ear-shaped surfaces of two different bones interlock, transmitting the weight of the spine to the legs

Sacroiliac joint — the auricular surfaces of sacrum and ilium.

Very strong posterior ligaments bind the two bones together with hardly any movement

Sacroiliac joint — the interosseous sacroiliac ligament.

Loosened by the hormone relaxin in pregnancy, allowing the pelvis to widen for childbirth

Sacroiliac joint — and the pubic symphysis.

Inflammation here, in a young adult with morning stiffness, is often the first sign of a condition that can fuse the spine

Sacroiliac joint — sacroiliitis in ankylosing spondylitis.

Two dimples on the lower back mark the level of the posterior superior iliac spines beside it

Sacroiliac joint — the 'dimples of Venus'.

Key ideas

  • Stability over movement: this is the bridge that carries load from the axial skeleton to the lower limb, so it is built to resist shear.
  • Movement at the joint is only a few degrees, and it tends to fuse with age.

Quick-fire facts

  • •OpenStax: the sacroiliac joint is the largely immobile joint between the auricular surface of the sacrum and the auricular surface of the ilium, supported by very strong ligaments.
  • •It is a synovial plane joint in the front part and a syndesmosis (fibrous joint) behind.
  • •The posterior interosseous sacroiliac ligament is very strong; it is often described as among the strongest ligaments in the body.
  • •Movements are tiny: nutation and counternutation of the sacrum.
  • •The hormone relaxin, released in pregnancy, softens the ligaments.
  • •Sacroiliitis is inflammation of the joint, seen in ankylosing spondylitis.
  • •Sacroiliac pain may be felt in the buttock and can mimic sciatica.

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