Sacroiliac joint

Sacroiliac joint
Sacroiliac joint of the male pelvis, posterior view
Human female pelvis, anterior view, with sacroiliac joint within red ellipse.
Details
Identifiers
Latin	articulatio sacroiliaca
TA	A03.6.03.001
FMA	21440
Anatomical terminology []

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 a strong, weight transferral 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.

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 articular 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 ilial 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.

As we age the characteristics of the sacroiliac joint change. The joint's surfaces are flat or planar in early life but as we start walking, the sacroiliac joint surfaces develop distinct angular orientations and lose their planar or flat topography. They also develop an elevated ridge along the ilial 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.

The ligaments of the sacroiliac joint include the following:

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 interosseous ligaments are very strong ligaments. They are often stronger than bone, such that the pelvis may actually fracture before the ligament tears. The dorsal sacroiliac ligaments include both long and short ligaments. The long dorsal sacroiliac joint ligaments run in an oblique vertical direction while the short (interosseous) runs perpendicular from just behind the articular surfaces of the sacrum to the ilium and functions to keep the sacroiliac joint from distracting or opening. The sacrotuberous and sacrospinous ligaments (also known as the extrinsic sacroiliac joint ligaments) limit the amount the sacrum flexes.