Anatomy
The sacroiliac joints are two paired "kidney bean" or L-shaped joints having a small amount of movement (2–18 degrees, which is debatable at this time) that are formed between the articular surfaces of the sacrum and the ilium bones. The two sacroiliac joints move together as a single unit and are considered bicondylar joints (where the two joint surfaces move correlatively together). The joints are covered by two different kinds of cartilage; the sacral surface has hyaline cartilage and the ilial surface has fibrocartilage. The SIJ's stability is maintained mainly through a combination of only some bony structure and very strong intrinsic and extrinsic ligaments. As we age the characteristics of the sacroiliac joint change. The joint's surfaces are flat or planar in early life but as we start walking, the sacroiliac joint surfaces develop distinct angular orientations (and lose their planar or flat topography.) They also develop an elevated ridge along the ilial surface and a depression along the sacral surface. The ridge and corresponding depression, along with the very strong ligaments, increase the sacroiliac joints' stability and makes dislocations very rare. The fossae lumbales laterales ("dimples of Venus") correspond to the superficial topography of the sacroiliac joints.
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