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Sacroiliac joint

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The sacroiliac joint or SI joint is the joint between the sacrum, at the base of the spine and the ilium of the pelvis, which are joined by ligaments. It 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 individually but are highly variable from person to person.

Pain may be caused by sacroiliitis, or an inflammation of the sacroiliac joint(s), a potential cause of low back pain. With sacroiliitis, the individual may experience pain in the low back, buttocks and thighs, and may also have other symptoms of a rheumatic condition such as inflammation in the eyes or psoriasis. Another related condition of the sacroiliac joint is called sacroiliac joint dysfunction (also termed SI joint dysfunction). While SI joint dysfunction may cause low back and sometimes leg pain from inflammation of the sacroiliac joint, some believe that sacroiliac joint dysfunction (also called syndrome) differs from sacroiliitis. The origin of the dysfunction is thought to be a problem with the normal movement of the sacroiliac joints (too much or too little movement in the joint) while sacroilitis (literally inflammation of the sacroiliac joint) is usually caused by one of the connective tissue diseases (e.g. ankylosing spondylitis, psoriasis).

Anatomy

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 stability of the SIJs 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. 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.

Ligaments

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 certainly not as strong and well defined as are the posterior 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.

Physiology

Like most joints, the SI joints' function includes some shock absorption 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 (become close pack) 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 joints are:

  • Anterior innominate tilt of both innominate bones on the sacrum (where the left and right move as a unit)
  • Posterior innominate tilt of both innominate bones on the sacrum (where the left and right move together as a unit)
  • Anterior innominate tilt of one innominate bone while the opposite innominate bone tilts posteriorly on the sacrum (antagonistic innominate tilt) which occurs during gait
  • Sacral flexion (or nutation)
  • Sacral extension (or counter-nutation)

Symptoms & Signs

The following are symptoms/signs that maybe associated with an SI joint (SIJ) problem:

  • Mechanical SIJ dysfunction usually causes a dull ache.
  • The pain is usually no more than a mild to moderate ache around the dimple or posterior superior iliac spine (PSIS) region.
  • The pain may become worse and sharp whilst doing activities such as standing up from a seated position, or lifting the knee up to the chest during stair climbing.
  • Typically the pain is felt on one side or the other (unilateral pain) but can occasionally be bilateral.
  • Noticing frequent changes in body posture to avoid prolonged stress on the SIJ and ligaments.
  • When SIJ dysfunction is severe, there can be referred pain into the hip, groin and occasionally down the leg, but rarely below the knee.
  • Pain can be referred from the SIJ down into the buttock or back of the thigh, and rarely to the foot.
  • Low back pain and stiffness, often unilateral, that often increases with prolonged sitting or prolonged walking.
  • Pain may occur during sexual intercourse.
  • Occasionally there may be referred pain into the lower limb, which can be mistaken for "true" sciatica from a herniated disk.

Sacroiliac joint dysfunction is tested in many different ways, although the reliability of most individual tests have been shown to be low. Using them in combination often improves 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 test, provocation test do best when clustered together.

Description of the Gillet test:

  1. With the patient standing and the examiner sitting behind, the examiner's left thumb is placed over the most posterior portion of the left posterior superior iliac spine (PSIS) and the right thumb overlying the midline of the sacrum at the same level.
  2. Examiner asks the patient to flex the left hip and knee as much as they can with a minimum of 90 degrees of the hip flexion. Imagine making an "L" with the leg and thigh.
  3. A negative test finds the left thumb on the posterior superior iliac spine (PSIS) moving caudal (towards the tail) in relation to the right thumb on the sacrum.
  4. The thumb placements are reversed, and the patient is asked to raise the right leg in similar fashion.
  5. A positive finding occurs when the thumb on the PSIS does not move at all or moves cranially (towards the head) in relation to the thumb on the sacrum.
  6. The findings of this test are correlated with those of the standing flexion test. The Stork test is often used to determine if the left or right SI joints are restricted.
  7. If the patient has difficulty standing on one foot to perform the test, proprioceptive sensory motor balance deficit should be further evaluated.

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).

Pregnancy

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.

  • In the long-running Amos 'n' Andy radio program, the phrase "I thinks I done broke my sacro-crackerjack!" was a repeated laugh line, which, like that program's "holy mackerel!", found its way into common usage.
  • The band 10cc has a song called "The Sacro-Iliac" on their 1974 album Sheet Music
  • In their 1981 New Wave song "Rapture", the band Blondie mentions people dancing cheek to cheek, with their sacroiliacs touching.
  • In the 10th season episode of the animated television sitcom King of the Hill titled "A Portrait of the Artist as a Young Clown", a comedy teacher instructs his class (including Bobby Hill) that "sacroiliac" is a "funny-sounding body part" because it has three "funny letters": two "k" sounds and an "oi" sound.

See also

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