The Clinical Significance and Anatomy of the Sacroiliac Joint
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. 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.
Anatomical Structure and Identifiers
It is a synovial plane joint with irregular elevations and depressions that produce interlocking of the two bones. The sacroiliac joints are paired C-shaped or L-shaped joints formed between the articular surfaces of the sacrum and the ilium bones. The joint space is usually 0.5 to 4 mm. The joints are covered by two different kinds of cartilage; the sacral surface has hyaline cartilage and the iliac surface has fibrocartilage.
For clinical and anatomical reference, the following identifiers are associated with the joint:
- Latin: articulatio sacroiliaca
- MeSH: D012446
- TA98: A03.6.03.001
- FMA: 21440
The fossae lumbales laterales ("dimples of Venus") correspond to the superficial topography of the sacroiliac joints.
Functional Movement and Stability
Sacroiliac joints are capable of a small amount of movement (2–18 degrees, which is debatable at this time). However, most agree that only slight movements occur on these joints, characterized by the following specific ranges:
- Flexion-extension: 3 degrees range of motion
- Axial rotation: 1.5 degrees
- Lateral bending: 0.8 degrees
Like most lower extremity joints, one of the SI joints' functions is shock absorption for the spine, along with the job of torque conversion allowing the transverse rotations that take place in the lower extremity to be transmitted up the spine. The SI joint provides a "self-locking" mechanism (where the joint occupies or attains its most congruent position, also called the close pack position) that helps with stability during the push-off phase of walking.
Development and Aging
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.
Ligaments of the Sacroiliac Joint
The SIJ's stability is maintained mainly through a combination of only some bony structure and very strong intrinsic and extrinsic ligaments. The ligaments of the sacroiliac joint include the following:
- Anterior sacroiliac ligament: Relatively thin, often appearing as a slight thickening of the anterior joint capsule, and is less well defined than the posterior sacroiliac ligaments.
- Interosseous sacroiliac ligament: These are very short and run perpendicular from the iliac surface to the sacrum; they keep the articular surfaces from abducting or opening/distracting.
- Posterior sacroiliac (SI) ligaments: These can be further divided into short (intrinsic) and long (extrinsic). The long dorsal sacroiliac joint ligaments run in an oblique vertical direction.
- Sacrotuberous and sacrospinous ligaments: Also known as the extrinsic sacroiliac joint ligaments, these limit the amount the sacrum flexes.
The dorsal interosseous ligaments are very strong, and in some cases pelvic fractures may occur before the ligaments tear. Notably, the ligaments of the sacroiliac joint loosen during pregnancy due to the hormone relaxin; this loosening, along with that of the related symphysis pubis, permits the pelvic joints to widen during the birthing process.