Sacroiliac Joint

The sacroiliac joint, or SI joint, is the articulation between the sacrum at the base of the spine and the ilium of the pelvis. There is one sacroiliac joint on each side of the sacrum. Together, they form an important mechanical connection between the axial skeleton and the pelvic girdle, helping transfer loads between the trunk and the lower limbs .^1,2

The joint is formed between the ear-shaped, or auricular, surfaces of the sacrum and ilium. Its anterior portion forms a synovial articulation, while the posterior region is strongly connected by ligaments. The irregular, interlocking shapes of the joint surfaces also contribute to stability .^1,2

Sacroiliac joint movement is small compared with movement at joints such as the hip or shoulder. Its limited motions include nutation and counternutation, which describe small rotations of the sacrum relative to the ilium. This combination of limited mobility and substantial stability allows the joint to accommodate forces while maintaining the structural continuity of the pelvis .^1,2

Strong ligaments are central to sacroiliac joint stability. These include the interosseous, anterior, and posterior sacroiliac ligaments, with additional support from the sacrotuberous and sacrospinous ligaments. The sacroiliac joint should not be confused with the hip joint. The SI joint connects the sacrum to the ilium, whereas the hip joint connects the femur to the acetabulum of the pelvis .^1,2