Sacroiliac joint

Location: At the back of the pelvis, on each side, between the lateral part of the sacrum and the ilium of the hip bone.

Bones taking part

• Sacrum
• Hip bone (ilium)

Articulating surfaces

• Synovial part: the ear-shaped auricular surface on the lateral side of the sacrum with the auricular surface of the ilium; both are covered with articular cartilage.
• The auricular surfaces are irregular, with elevations and depressions that interlock and help lock the bones together.
• Fibrous part: the rough sacral tuberosity (behind the auricular surface of the sacrum) and the iliac tuberosity (behind the auricular surface of the ilium), joined by the interosseous sacroiliac ligament.

Structural

Compound joint: an anterior synovial joint between the auricular surfaces, and a posterior fibrous joint (syndesmosis) where the sacral and iliac tuberosities are united by the interosseous sacroiliac ligament.

Functional

Amphiarthrosis in practice (only slight movement), although its anterior part is structurally synovial.

Synovial subtype

Plane synovial (anterior part only); the irregular, interlocking surfaces allow only very slight movement.

Capsule & cavity

The synovial part has a fibrous capsule lined by synovial membrane. Anteriorly the capsule is thickened as the thin anterior sacroiliac ligament. The joint cavity may become partly obliterated by fibrous tissue in older people.

• Standing: transfers the weight of the head, trunk and upper limbs from the vertebral column (through the sacrum) to the hip bones, then to the femora.
• Sitting: weight passes from the sacrum across the joint to the ilia and down to the ischial tuberosities.
• Walking and running: the slight 'give' of the joint and its strong ligaments act as a stress-relieving link between spine and lower limbs.
• Balance and posture: by locking the sacrum firmly between the hip bones it provides a stable base for the vertebral column.
• Childbirth: softening of its ligaments in late pregnancy allows slight extra movement.

Stability vs mobility: Stability is far more important than mobility here. The sacrum is wedged and suspended between the ilia; the interlocking auricular surfaces, the very strong interosseous and posterior sacroiliac ligaments, and the sacrotuberous and sacrospinous ligaments together prevent the sacrum from being pushed forward and down by body weight. The price is that movement is very limited.

• Sacroiliitis (inflammation of the joint) is a common early feature of ankylosing spondylitis and can cause low back and buttock pain.
• In late pregnancy hormonal softening of the pelvic ligaments increases movement at the joint, and the joint can be a source of pelvic girdle pain.
• The joint may become partly fused with age, which further reduces its small movement.
• Sacroiliac pain may be felt in the buttock and can be confused with lumbar disc or hip problems.

• Name it a compound joint: anterior synovial (plane) + posterior syndesmosis.
• Know which surfaces meet: auricular surfaces of sacrum and ilium; the tuberosities are joined by the interosseous ligament.
• The interosseous sacroiliac ligament is the strongest; the anterior ligament is thin.
• Explain weight transfer: vertebral column → sacrum → ilia → femora (standing) or ischial tuberosities (sitting).
• Nutation and counternutation are small movements, described qualitatively.
• Accessory ligaments: sacrotuberous, sacrospinous, iliolumbar.

• Calling it a purely synovial joint — it is compound, with a fibrous posterior part.
• Calling it a freely movable joint — movement is very slight.
• Saying the sacrum articulates with the ischium — it articulates with the ilium.
• Naming the anterior sacroiliac ligament as the strongest — it is the thinnest; the interosseous ligament is strongest.
• Giving muscles that 'move' the sacroiliac joint — none act on it specifically.