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Grading sacroiliitis
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Sacroiliac joints

The sacroiliac joints are built to be stable during the transmission of forces from the body to the lower extremities. This stability is obtained by obliquely orientated undulating joint facets, surrounding joint ligaments, and also strong external ligaments and muscles.

The joint is composed of two compartments: a C-shaped cartilaginous part that lies inferiorly/anteriorly and a superior/posterior ligamentous part containing strong inter-osseous ligaments. The cartilaginous joint facets are convex anteriorly and are usually narrower in females than in males, although the joint varies greatly in size, shape and contour. The cartilaginous portion of the joint is without synovia (a symphysis) proximally whereas there are small synovial recesses at the distal part of the joint. The anatomy of the middle part is rather variable with a variety of normal variations giving raise to pitfalls at MR imaging.





Macroscopic anatomy and function of the sacroiliac joints
The images show the joint localisation and stabilising muscles in addition to weight load in the upright position.
Anatomical portions

Images delineating the joint portions on a axial slice (left). There is a cartilaginous portion ventrally (VCJP), the transition zone in the middle (TZ) and the ligamentous portion posterior (DLP).

Microscopic anatomy - proximal joint portion

Proximal part of the cartilaginous portion of the joint with strong ligaments connecting the joint facets anterior and proximally (VSIL = ventral sacroiliac ligament and PSIL = proximal sacroiliac ligament). Posteriorly the ligaments merge with the cartilage forming fibro-cartilaginous like tissue. There is no synovia in this region. I = ileum; S = sacrum; FC = fibrocartilage.

Microscopic anatomy - distal joint portion

In the distal part there are small recesses lined by synovia both anterior and posterior (arrows).

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