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Articulations of pelvis. Anterior view.
Articulations of pelvis. Posterior view.
Articulations of pelvis. Anterior view.
Articulations of pelvis. Posterior view.
The sacroiliac joint or SI joint is the joint in the bony pelvis between the sacrum and the ilium of the pelvis, which are joined 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 bearing synovial 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.
The sacroiliac joints are two paired "kidney bean" or L-shaped joints having 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 two sacroiliac joints move together as a single unit and are considered bicondylar joints (where the two joint surfaces move correlatively together). 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 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 extrinsic sacroiliac joint ligaments (the sacrotuberous and sacrospinous ligaments) limit the amount the sacrum flexes (or nutates).
The ligaments of the sacroiliac joint become loose during pregnancy due to the hormone relaxin; this loosening allows widening of the pelvic joints during the birthing process, especially the related symphysis pubis. The long SI ligaments may be palpated in thin persons for pain and compared from one side of the body to the other; however, the reliability and the validity of comparing ligaments for pain have currently not been shown. The interosseous ligaments are very short and run perpendicular from the iliac surface to the sacrum, they keep the articular surfaces from abducting or opening/distracting.
Like most lower extremity joints, one of the SI joints' function is shock absorption (depending on the amount of available motion at the sacroiliac joint) 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, like all lower extremity joints, 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. The joint locks (or rather becomes close packed) on one side as weight is transferred from one leg to the other, and through the pelvis the body weight is transmitted from the sacrum to the hip bone.
The motions of the sacroiliac joint
The sacroiliac joints like all spinal joints (except the atlanto-axial) are bicondylar joints, meaning that movement of one side corresponds to a correlative movement of the other side.
Sacroiliitis refers to inflammation of one or both sacroiliac joints, and is one cause of unilateral low back pain. With sacroiliitis, the individual may experience pain in the low back, buttock or thigh, depending on the amount of inflammation. Common problems of the sacroiliac joint are often called sacroiliac joint dysfunction (also termed SI joint dysfunction; SIJD). Sacroiliac joint dysfunction generally refers to pain in the sacroiliac joint region that is caused by abnormal motion in the sacroiliac joint, either too much motion or too little motion. It typically results in inflammation of the SI joint, or sacroiliitis.
The following are symptoms/signs that may be associated with an SI joint (SIJ) problem:
Sacroiliac joint dysfunction is tested using provocative and nonprovocative maneuvers. Nonprovocative sacroiliac joint examination maneuvers would include Gillet Test, prone knee flexion test, supine long sitting test, standing flexion test, and seated flexion test. There is a lack of evidence that these sacroiliac joint mobility maneuvers detect motion abnormalities. 
Given the inherent technical limitations of the visible and palpable signs from these sacroiliac joint mobility maneuvers another broad category of clinical signs have been described called provocative maneuvers. These maneuvers are designed to reproduce or increase pain emanating within the sacroiliac joint. When the provocative maneuvers reproduce pain along the typical area raises suspicion for sacroiliac joint dysfunction. However no single test is very reliable in the diagnosis of sacroiliac joint dysfunction. It is important to remember true neurogenic weakness, numbness, or loss of reflex should alert the clinician to consider nerve root pathology.
The current gold standard for diagnosis of sacroiliac joint dysfunction emanating within the joint is sacroiliac joint injection confirmed under fluoroscopy or CT-guidance using a local anesthetic solution. The diagnosis is confirmed when the patient reports a significant change in relief from pain and the diagnostic injection is performed on 2 separate visits. Published studies have used at least a 75 percent change in relief of pain before a response is considered positive and the sacroiliac joint deemed the source of pain. 
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The hormonal changes of menstruation, pregnancy, and lactation can affect the integrity of the ligament support around the SIJ, which is why women often find the days leading up to their period are when the pain is at its worst. During pregnancy, female hormones are released that allow the connective tissues in the body to relax. The relaxation is necessary so that during delivery, the female pelvis can stretch enough to allow birth. This stretching results in changes to the SIJs, making them hypermobile—extra or overly mobile. Over a period of years, these changes can eventually lead to wear-and-tear arthritis. As would be expected, the more pregnancies a woman has, the higher her chances of SI joint problems. During the pregnancy, micro tears and small gas pockets can appear within the joint.
Muscle imbalance, trauma (e.g., falling on the buttock) and hormonal changes can all lead to SIJ dysfunction. Sacroiliac joint pain may be felt anteriorly, however, care must be taken to differentiate this from hip joint pain.
Women are considered more likely to suffer from sacroiliac pain than men, mostly because of structural and hormonal differences between the sexes, but so far no credible evidence exists that confirms this notion. Female anatomy often allows one less sacral segment to lock with the pelvis, and this may increase instability.
This article uses anatomical terminology; for an overview, see anatomical terminology.
21. Huijbregts P (2004). "Sacroiliac joint dysfunction: Evidence-based diagnosis". Orthopaedic Division Review. p18–44. May/June.