Sacroiliac joint

Dr Henry Knipe et al.

The sacroiliac (SI) joint is a synovial and fibrous joint between ilium and the sacrum. It has little movement and its main function is to transfer weight between the axial and lower appendicular skeletons. The SI joint is a symmetrical joint (i.e. is paired) with an oblique coronal orientation and is located at the S1-3 level 1-3.

  • location: the synovial articulation between the sacral and iliac auricular surfaces (facies)
  • movement: minimal; gliding and rotation
  • ligaments and tendons: anterior, posterior and interosseous sacroiliac ligaments; iliolumbar, sacrotuberous and sacrospinous ligaments
  • relations: piriformis muscle, lumbosacral trunk; internal, external and common iliac veins

Movement at the sacroiliac joint is minimal and is limited to gliding and rotation. Most movement is experienced during pregnancy due to laxity of ligaments and the joint typically fuses after the age 50 years 1,2

The sacroiliac joint is formed by the irregularly-shaped auricular surfaces of the sacrum and the ilium 1-2. The upper third is a syndesmosis. The lower two thirds are lined by articular cartilage, although only the lower third is lined by synovium, while the middle third resembles a symphysis. Unlike other synovial joints, the articular facets of the sacroiliac joint are lined by fibrocartilage 4. Of note, cartilage is more abundant anteriorly and thinner on the iliac side.

Movement and stability is also conferred by a few muscles 3:

Being a synovial joint it is surrounded by a capsule. Stability is conferred by the irregular articular surfaces but is actually mainly through extra-capsular ligaments 1-2:

Primarily anteriorly:

  • corresponds with arterial blood supply

Via branches from the lumbar and sacral plexuses 3:

Anatomy: Spine
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Article information

rID: 24607
Section: Anatomy
Synonyms or Alternate Spellings:
  • SI joint
  • Sacro-iliac joint

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Cases and figures

  • Case 1: normal plain X-ray appearance
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  • Case 2: normal CT appearance
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  • Case 3: normal MR appearance
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  • Case 4: accessory sacroiliac joints
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