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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 ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side. The joint is a strong, weightbearing synovial joint with irregular elevations and depressions that produce interlocking of the two bones.The human body has two sacroiliac joints, a left and a right joint, that often match each other but are highly variable from person to person.
The sacroiliac joints are two paired “kidney bean” or L-shaped synovial joints that have minimal motion (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 SIJs stability are maintained mainly through a combination of both 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. 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.
Depending on the reference source cited, the anterior ligament may be described as just a slight thickening of the anterior joint capsule. The anterior ligament is thin and and not as well defined as are the posterior sacroiliac joint ligaments.
The posterior sacroiliac (SI) ligaments can be further divided into short (intrinsic) and long (extrinsic).The dorsal interosseous ligaments are very strong ligaments. This ligament is even stronger than bone; such that the pelvis will usually fracture before the ligament tears. The dorsal sacroiliac ligament runs perpendicular from just behind the articular surfaces of the sacrum to the ilium and function 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 short ligaments (e.g. interosseous) cannot be assessed, since they are located deep inside the pelvis.
Like most joints, the SI joints’ function includes some shock absorption (depending on the amount of available motion at the sacroiliac joint) for the spine, along with 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 pushoff phase of walking.The joint locks (not really locks but 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
Pain is usually caused by sacroiliitis an inflammation of one of the sacroiliac joint(s), which is a frequent cause of unilateral low back pain. With sacroiliitis, the individual may experience pain in the low back, buttock or thigh. More commonly problems of the sacroiliac joint are called sacroiliac joint dysfunction (also termed SI joint dysfunction). The putative cause of sacroiliac joint dysfunction is thought to be a problem in the movements of the sacroiliac joints (either too much or too little movement in the joint or when the innominate bones assume an antagonstic position when they normally should be symmetrical)
The following are symptoms/signs that maybe associated with an SI joint (SIJ) problem:
Sacroiliac joint dysfunction is tested in many different ways, although the reliability of most individual tests have been shown to be low. Using tests in combination will often improve their reliability. Commonly used tests used to identify dysfunction include the Gillet or also called Stork Test, the prone knee flexion test, the Supine Long Sitting test, the standing flexion test, and the seated flexion test. Another group of tests are called provocation tests, although these tests are not used to determine the type of sacroiliac joint dysfunction, they have been shown to be both reliable and valid for helping to determine the likely source of back pain. Provocation tests include: the posterior shear, central posterior/anterior pressure on the sacrum, Gaenslen’s test, sacroiliac joint compression and distracting (gapping) test. Like most all sacroiliac joint tests, provocation tests do best when clustered together.Description of the Gillet test:
Tests should be interpreted carefully since false positive and false negative test results are common due to their low reliability. A method that can reduce the findings of false positive and false negatives is to cluster the individual tests together. Finding 3 or 4 tests for a specific type of sacroiliac joint dysfunction, for example a right posterior innominate, reduces the likelihood of false results. Adding a cluster of tests, along with the presence of absence of other sacroiliac joint symptoms and signs, can reduce the risk of spurious findings. For the examiner to achieve success, however, the tests must agree with each other, that, is the test results must agree on finding the same problem (eg. a right posterior innominate tilt). There are many other tests available.Passive mobility tests can be used to evaluate the hip, sacroiliac joint, and lumbar spine (again clinicians must be careful because of the low reliability of many of these tests). Some clinicians recommend the use of spring tests. These are performed with various positions, such as having the person lie prone, supine, sitting, side-lying, prone extension and prone flexion (also called Muslim Prayer Position).
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.
Trauma, muscle imbalance, 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. Also, when the front part of the pelvis moves down relative to the spine, it stretches the psoas muscle. The ligaments helping to stabilize the SIJ can become lax and this, together with increased load on the spine due to the pregnancy, can cause altered SI joint mechanics and pain. Any type of back or sacroiliac problem that causes excessive movement of the pelvis can result in excessive movement in the pubic symphysis and its ligaments. Sometimes an obvious limp is present due to one or both of the joints locking. There is a relation between asymmetric laxity of the sacroiliac joints and pregnancy related pelvic girdle pain. This condition can begin either pre- or post-partum.
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.
Because “sacroiliac” is a colloquially peculiar-sounding word, it often has been used for humorous or rhyming purposes in popular culture.
Musculoskeletal Consumer Review http://en.wikipedia.org/w/index.php?title=Pelvic_girdle_pain&action=submit
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