The sacroiliac (SI) joint is the joint connecting the sacrum to the left and right iliac bones that compose the pelvis. The sacrum is a large, triangular bone at the base of the spine. It is formed by the fusion of five vertebrae. The SI joints connect the spine to the pelvis. One of the functions of SI joints is to absorb shock on the upper body, pelvis, and legs. They also help stabilize the body and maintain an upright posture.
SI joints have to bear the full weight of the upper body when you stand erect. This puts stress on the joints and causes wearing of the cartilage of the SI joint, and over time leads to degenerative arthritis.