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The sacroiliac joint or SI joint (SIJ) is the joint between the sacrum and the ilium bones of the pelvis, which are connected by strong ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side. The joint is strong, supporting the entire weight of the upper body.
Magnetic resonance images of sacroiliac joints. Shown are T1-weighted semi-coronal magnetic resonance images through the sacroiliac joints (a) before and (b) after intravenous contrast injection. Enhancement is seen at the right sacroiliac joint (arrow, left side of image), indicating active sacroiliitis. This patient had psoriatic arthritis.
On either side of the base is a large projection known as an ala of sacrum and these alae (wings) articulate with the sacroiliac joints. The alae support the psoas major muscles and the lumbosacral trunk which connects the lumbar plexus with the sacral plexus. In the articulated pelvis, the alae are continuous with the iliac fossa.
At one point, the sacroiliac joints were thought to be a major source of low back pain, but more accurate methods of diagnosis have proved that not to be the case.
The posterior sacroiliac ligament is situated in a deep depression between the sacrum and ilium behind; it is strong and forms the chief bond of union between the bones. It consists of numerous fasciculi , which pass between the bones in various directions.
Sacroiliac joint dysfunction is an outcome of either extra-articular dysfunction or from intraarticular dysfunction. SI joint dysfunction is sometimes referred to as "sacroiliac joint instability" or "sacroiliac joint insufficiency" due to the support the once strong and taut ligaments can no longer sustain.
Magnetic resonance images of sacroiliac joints: psoriatic arthritis. Shown are T1-weighted semi-coronal magnetic resonance images through the sacroiliac joints (a) before and (b) after intravenous contrast injection. Enhancement is seen at the right sacroiliac joint (arrow), indicating active sacroiliitis.
Magnetic resonance images of sacroiliac joints. Shown are T1-weighted semi-coronal magnetic resonance images through the sacroiliac joints (a) before and (b) after intravenous contrast injection. Enhancement is seen at the right sacroiliac joint (arrow, left side of the image), indicating active sacroiliitis.