Sacroiliac Joint Sprain and Dysfunction

There are two Sacroiliac (SI) joints. They connect both sides of the sacrum to the iliac
part of the pelvic bones, forming the ring of the pelvis. The sacrum is usually formed
by five vertebrae fused together (though it can be four or six).  The SI joints are big
and very strong and are synovial (fluid filled) joints. They are held together by thick
and strong ligaments. They are subjected to a small rocking movement in opposite
directions to each other when walking. They are subjected to twisting forces when
the spine twists. They are under compressive forces when standing or walking, and
extreme forces when landing after a jump. The SI joints are ‘shock absorbers’, and
transfer forces from the axial skeleton to the legs, and from the legs to the axial
skeleton. In women, a large movement of the SI joints are needed to facilitate
childbirth. No single muscle crosses the joints and moves the joints as a ‘prime
mover’; instead, the joints are moved as a result of complex musculoskeletal
movements such as walking.

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