Mobility of the sacro-iliac joint
- The auricular surfaces of the sacrum and the hip bone have cartilages, and an articular cavity exists between them. Thus, although it is a joint in the narrow sense (synovial joints), the sacro-iliac joint's joint capsule is so tightly reinforced by the ventral sacro-iliac ligament, interosseous sacro-iliac ligament, and dorsal sacro-iliac ligament that it can hardly be moved.
- In cows and other animals, progesterone act on these ligaments to make them more flexible and facilitate childbirth. This may also be the case in humans.