The sacroiliac joint (SIJ) is a highly complex joint that provides stability and support to the upper body ( Fig. 1.1 ). The specific structure and tightness of the fibrous apparatus of the SIJ results in its limited mobility. The importance of the SIJ as a stress reliever between the trunk and lower limbs has been emphasized. This joint ensures that the pelvic girdle is not a sold ring of bones that can easily break under routine stresses it might be subject to. The SIJ is known to be the largest axial spinal joint in the body and is approximately 17.5 cm 2 . The SIJ is a true diarthrodial joint that is more mobile in youth than later in life. The female pelvis is also more mobile to accommodate pregnancy and parturition. , This joint has at multiple junctures a fibrous joint capsule that contains a thick synovial fluid, cartilaginous surfaces, and numerous ligamentous connections. It is different from other synovial joints in that the iliac articulation is made of fibrocartilage rather than hyaline cartilage.
Bony anatomy
There is great variance in shape and size of the bony anatomy of the SIJ among individuals ( Fig. 1.2 ). From infancy to adulthood, notable changes in the joint occur. The articular surface of the sacrum is generally concave, and the iliac surface is predominantly convex. The SIJ has numerous ridges and grooves compared with a typical synovial joint. This characteristic of the joint minimizes movement, further enhancing stability. The ventral aspect of the SIJ frequently has defects that allows fluid in the joint to leak out to the surrounding structures.
Sacralization, or fusion of the fifth lumbar vertebrae into the body of the sacrum, occurs in about 6% of American adults. Fusion between the L5 and S1 vertebrae can occur at one or more locations, such as between transverse processes, vertebral bodies, or facet joints. Accessory SIJs have been described as extracapsular articulations for biomechanical enhancement.
Ligaments and muscles supporting the sacroiliac joint
The SIJ is mainly designed to support stability and weight bearing with only small degrees of rotation and translation allowed. , Joint stability is enforced by multiple ligaments that include the anterior sacroiliac ligament (ASL), posterior sacroiliac ligament (PSL), sacrospinous ligament (SSL), sacrotuberous ligament (STL), and interosseous ligament (which is considered the strongest) , ( Figs. 1.3 and 1.4 ). The interosseous sacroiliac ligament encloses the axial joint and fills the spaces dorsal and caudal to the synovial joint. It has the most extensive bony attachment and volume among all sacroiliac ligaments. The axial joint is therefore difficult to access because of the irregular contour, extensive fibrous apparatus, and individual variations. Whereas the PSL has the most influence on the joint mobility, the ASL has very limited effect. The iliolumbar (IL) ligament is a large fan-shaped structure that extends from the transverse processes of the lower two lumbar vertebrae to the iliac crest and the SIJ capsule. The main function of the IL ligament is to restrict movements at the lumbosacral junction, especially side bending.
