<Tr> <Th> FMA </Th> <Td> 21440 </Td> </Tr> <Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> 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 . </P> <P> 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 . 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 . </P>

What kind of joint is the si joint