The sacroiliac joint or SI joint (SIJ) is the joint between the sacrum and the ilium bones of the pelvis, which are connected by strong ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side. The joint is strong, supporting the entire weight of the upper body. It is a synovial plane joint with irregular elevations and depressions that produce interlocking of the two bones. The human body has two sacroiliac joints, one on the left and one on the right, that often match each other but are highly variable from person to person. Structure Sacroiliac joints are paired C-shaped or L-shaped joints capable of a small amount of movement (2–18 degrees, which is debatable at this time) that are formed between the auricular surfaces of the sacrum and the ilium bones. However most agree that there are only slight movements occur on these joints, with only 3 degrees range of motion during flexion-extension, followed by 1.5 degrees axial rotation, and 0.8 degrees lateral bending. The joints are covered by two different kinds of cartilage; the sacral surface has hyaline cartilage and the iliac surface has fibrocartilage. The SIJ's stability is maintained mainly through a combination of only some bony structure and very strong intrinsic and extrinsic ligaments. The joint space is usually 0.5 to 4 mm. Aging changes the characteristics of the sacroiliac joint. The joint's surfaces are flat or planar in early life. Once walking ability is developed, the sacroiliac joint surfaces begin to develop distinct angular orientations and lose their planar or flat topography. They also develop an elevated ridge along the iliac surface and a depression along the sacral surface. The ridge and corresponding depression, along with the very strong ligaments, increase the sacroiliac joints' stability and makes dislocations very rare. The fossae lumbales laterales ("dimples of Venus") correspond to the superficial topography of the sacroiliac joints. Ligaments The ligaments of the sacroiliac joint include the following: Anterior sacroiliac ligament Interosseous sacroiliac ligament Posterior sacroiliac ligament Sacrotuberous ligament Sacrospinous ligament The anterior ligament is not much of a ligament at all and in most cases is just a slight thickening of the anterior joint capsule. The anterior ligament is thin and not as well defined as the posterior sacroiliac ligaments. The posterior sacroiliac (SI) ligaments can be further divided into short (intrinsic) and long (extrinsic). The dorsal