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.