Joint · Pelvis

Sacroiliac joint

Articulatio sacroiliaca

Where the sacrum wedges between the two hip bones at the back of the pelvis. It is an unusual joint: the front part is a true synovial joint, while the back part is a dense mass of ligament binding the bones together. It is built for load transfer, not for movement — everything from the upper body passes through it into the legs — so its ridged, interlocking surfaces and enormous ligaments give it very little available motion. The movement it does have is real but small, and it is felt just below and to the side of the dimples of the low back.