The SI joints, or sacroiliac joints, connect the sacrum with the pelvis, according to Spine-health. The sacrum is the triangle-shaped bone found at the bottom of the spine, and the pelvis is part of the hip joint.
Though it is a joint, the SI joint does not move much and is very strong due to dense, tough connective tissue, explains InnerBody. However, the joint moves slightly to allow for childbirth. It is covered with cartilage that is roughened to prevent the bones from moving too much, though synovial fluid lubricates the joints and allows for slight movement.
The SI joints are held together by two ligaments, notes InnerBody. The anterior sacroiliac ligament, which is wide but thin, holds the pelvis and the sacrum together on the inside of the pelvis. The posterior sacroiliac ligament holds the back of the sacrum to the back of the pelvis and the front of the iliac spine. This ligament is much stronger and thicker than the anterior sacroiliac ligament.
The SI joint can be a locus of low back pain, states Spine-health. Doctors believe this happens when the joint is misaligned or inflamed. When this occurs, the patient feels pain in the buttocks or very low on one side of the back. This disorder is most often found in women.