Sacroiliac joint

the articulation between the sacrum and ilium. The articulating surfaces of these bones are irregular and allow interdigitation, which results in greater strength of the joint as well as restricted motion. Also, see sacroiliac joint.

2. Pathology

The sacroiliac joints are affected in numerous pathologic conditions. The joints may be involved unilaterally, bilaterally and symmetrically, or bilaterally and asymmetrically. Almost exclusively the synovial part of the joint (the lower one half or two thirds of the interosseous space between sacrum and ilium) is affected to a greater degree than the ligamentous portion (that area above the synovium-lined space). Inflammation (sacroiliitis) is one of the most common abnormalities of this joint and can occur under numerous circumstances.

Sacroiliitis is one of the hallmarks of ankylosing spondylitis. In the later stages of the disease the alterations are almost invariably bilateral and symmetric. They predominate in the ilium. Irregular bony bridges may traverse the articular cavity, later resulting in complete ankylosis.

In degenerative joint disease cartilage erosion, partial or complete fibrous ankylosis of the joint cavity, subchondral eburnation and osteophytes may become apparent. Radiographic manifestations include diffuse loss of joint space and focal or diffuse, well-defined subchondral bone sclerosis. Erosion of subchondral bone is infrequent, but osteophytes are common and may occur at any level in the joint. CT scanning can demonstrate the bone excrescences that bridge the articular cavity. An additional radiographic manifestation of degenerative sacroiliac joint disease is a vacuum phenomenon, which is a nonspecific finding. The gas that produces the radiolucent line may extend from the joint into cystic lesions in the subchondral space (pneumatocysts).

Infections of the sacroiliac joint may spread haematogenously, by contamination from a contiguous suppurative focus, or by direct implantation. They may also develop after surgery. Pyogenic infection of the sacroiliac joint is generally unilateral and is accompanied by fever, local pain and tenderness, sciatica and a limp. Progressive changes are accompanied by erosions. Surrounding condensation of bone is of variable degree. With treatment, intra-articular osseous fusion may be encountered. Scintigraphy, CT scanning, and MR imaging are valuable in the diagnosis of septic sacroiliitis, in the detection of soft tissue extension of the infection, and as an aid to aspiration and biopsy techniques.

Sacroiliac joint abnormalities have also been described in familial Mediterranean fever, relapsing polychondritis, Behcets syndrome, gout, alkaptonuria, psoriatic arthritis, inflammatory bowel diseases, hyperparathyroidism, Reiters syndrome, rheumatoid arthritis and juvenile chronic arthritis. Radiation therapy and immobilization may also produce alterations in the sacroiliac joints. Osteitis condensans ilii produces bilateral and symmetric alterations in young women consisting of well-defined triangular sclerosis of the inferior aspect of the ilium.

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