Structure and function
The sacroiliac joint (SI) is the only bony connection between the horse’s hindlimb and the vertebral column. The joint is composed of two major bones: the wing of the ilium, part of the pelvis, and the wing of the sacrum, part of the spine. Two SI joints sit on either side of the spine. The joints’ main function is to connect the hind limbs to the rest of the skeleton with minimal movement in the joint itself. It transfers forces from the hindlimb during weight bearing through to the vertebral column. The ligaments and muscles surrounding the joint play an essential role in supporting the joint.