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Sacroiliac joint dysfunction or incompetence 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 sacroiliac joint, and can be debilitating.
The sacroiliac joint is a true diarthrodial joint that joins the sacrum to the pelvis. The sacrum (tailbone) connects on the right and left sides of the ilia (pelvic bones) to form the sacroiliac joints. The pelvic girdle is made up of two innominate bones (the iliac bones) and the sacrum. The innominate bones join in the front of the pelvis to form the pubic symphysis, and at back of the sacrum to form the sacroiliac (SI) joints. Each innominate bone (ilium) joins the femur (thigh bone) to form the hip joint; thus the sacroiliac joint moves with walking and movement of the torso.
In this joint, hyaline cartilage on the sacral side moves against fibrocartilage on the iliac side. The sacroiliac joint contains numerous ridges and depressions that function in stability. Studies have documented that motion does occur at the joint; therefore, slightly subluxed and even locked positions can occur.
Muscles and ligaments surround and attach to the SI joint in the front and back, primarily on the ilial or sacral surfaces. These can all be a source of pain and inflammation if the SI joint is dysfunctional. The sacroiliac joint is highly dependent on its strong ligamentous structure for support and stability. The most commonly disrupted and/or torn ligaments are the iliolumbar ligament and the posterior sacroiliac ligament. The ligamentous structures offer resistance to shear and loading. The deep anterior, posterior, and interosseous ligaments resist the load of the sacrum relative to the ilium. More superficial ligaments (e.g., the sacrotuberous ligament) react to dynamic motions (such as straight-leg raising during physical motion). The long dorsal sacroiliac ligament can become stretched in periods of reduced lumbar lordosis (e.g., during pregnancy).
Common symptoms include lower back pain, buttocks pain, sciatic leg pain, groin pain, hip pain (for explanation of leg, groin, and hip pain, see referred pain), urinary frequency, and "transient numbness, prickling, or tingling." Pain can range from dull aching to sharp and stabbing and increases with physical activity. Symptoms also worsen with prolonged or sustained positions (i.e., sitting, standing, lying). Bending forward, stair climbing, hill climbing, and rising from a seated position can also provoke pain. Pain is reported to increase during sexual intercourse and menstruation in women. Patients with severe and disabling sacroiliac joint dysfunction can suffer from insomnia and depression.
Many large and small muscles have relationships with the ligaments of the sacroiliac joint including the piriformis (see “piriformis syndrome”, a condition often related with sacroiliac joint dysfunction), biceps femoris, gluteus maximus and minimus, erector spinae, latissimus dorsi, thoracolumbar fascia, and iliacus. Any of these muscles can be involved or spasm with a painful and dysfunctional sacroiliac joint. The SI joint is a pain-sensitive structure richly innervated by a combination of unmyelinated free nerve endings and the posterior primary rami of spinal segments L2-S3. The wide possibility of innervation may explain why pain originating from the joint can manifest in so many various ways, with different and unique referral patterns (see “referred pain”) for individual patients. Patients with sacroiliac joint dysfunction can also develop tightness and dysfunction in the hamstring, quadriceps, iliotibial tract (see “iliotibial band syndrome”) and hip flexors, including the psoas muscle. Individuals with severe and long-standing sacroiliac joint dysfunction can develop muscle deconditioning and atrophy throughout the body due to limitation of activities and exercise that bring about pain in the low back.
SI joint dysfunction is sometimes referred to as "sacroiliac joint instability" or "sacroiliac joint insufficiency" due to the lack of support the once strong and taut ligaments can no longer sustain. When the joint is hypermobile or loose, it is classified as an extra-articular dysfunction because abnormal joint movement and alignment is a consequence of weakened, injured, or sprained ligaments, while the joint itself is structurally normal and healthy. The sacroiliac joint itself often will not show degenerative changes, such as arthritis, until many years of the dysfunction being allowed to continue. Injury to the ligaments that hold the sacroiliac joints in proper support is thought to be caused by a torsion or high impact injury (such as an automobile accident) or a hard fall, resulting in the hypermobility. As many as 58% of patients diagnosed with sacroiliac joint pain had some inciting traumatic injury based on clinical examination findings. The joint that was once stabilized by strong ligaments, now overly stretched, sprained, or torn, will move beyond its normal range. This is thought to result in the ilium and sacral surfaces "locking" in an incongruent or asymmetrical fashion (one innominate bone is tilted anteriorly; the other innominate bone is tilted posteriorly) causing pain that can be debilitating.
Hormone imbalances, particularly those associated with pregnancy and the hormone relaxin, can also cause a ligamentous laxity resulting in the weakening of the sacroiliac structure.  During pregnancy, relaxin serves as nature's way of allowing the female pelvis to achieve distention of the birthing canal. Pelvic joint pain in post pregnancy women is thought to be derived from the inability of the stretched out ligaments to return to normal tautness. Women who have delivered large babies or who have had extended labors also are prone to developing chronic sacroiliac joint pain and instability. Concern has been expressed that specific testing is necessary to determine if peri-partum or post-partum hypermobility is present because there is a case in which the patient had all classical signs and symptoms of instability, yet micromotion testing revealed the joint to be stuck, and mobilization was effective in one visit. Pain, including a long history of interstitial cystitis, and peri-partum uriniary frequency were much improved and she had a very easy delivery. It is suggested that the stretch receptors of the bladder and uterus provoked the symptoms which were interpreted as "instability."
In some people, the sacroiliac joints reverse the normal concave-convex 'locking' relationship, which can lead to rotational misalignment. The variation in joint configuration results in some sacroiliac joints being inherently weaker or more prone to misalignment. Certain biomechanical or muscle length imbalances may ultimately predispose a person to sacroiliac dysfunction and pain. Likely, this is a result of altered gait patterns and repetitive stress to the SI joint and related structures. These conditions exist in persons with leg-length inequality, scoliosis, a history of polio, poor-quality footwear, and hip osteoarthritis. There is also a notable incidence of lumbar spinal fusion patients that present with sacroiliac pain and hypermobility, potentially due to the adjacent lumbar joints being fixed and unable to move. Clinical studies have found up to 75% of post-lumbar fusion patients develop SI joint degeneration within five years of surgery.
Hypomobility (too little movement) of the sacroiliac joint is an intra-articular disorder in which the joint locks due to wearing down with age or degenerative joint disease. Hypomobility of this kind can also occur with an inflammatory disease such as ankylosing spondylitis, rheumatoid arthritis, or an infection.
Perhaps the biggest reason for misdiagnosis or lack of diagnosis of sacroiliac joint dysfunction is based on the inability of common radiological imaging to discern the disorder. Diagnostic testing, such as X-ray, CT scan, or MRI, do not usually reveal abnormalities; therefore, they cannot reliably be used for diagnosis of sacroiliac joint dysfunction. There is a new definitive imaging test SPECT/CT which can detect sacroiliac joint dysfunction. There is also a lack of evidence that sacroiliac joint mobility maneuvers (Gillet, Standing flexion test, and Seated Flexion test) 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.
A clinician (i.e., a spine surgeon, orthopedic surgeon, sports medicine doctor, physical therapist, physiatrist, osteopath or chiropractor) can develop a probable diagnosis of sacroiliac joint dysfunction by using a hands on approach through palpating the painful areas and performing the following provocative maneuvers below:
Cautious interpretation is warranted because there are no biomechanical studies showing that the Thigh Thrust test isolates forces in the SIJ when performed at 90 degrees and due to intra-individual variation in body type, hip flexibility, general flexibility of the trunk and pelvis. In all the tests, 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. However, several other injection studies have compared intra-articular with extra-articular injection, and indicate that the ligament injection behind the joint is oftentimes superior to injection in the joint and seems to be a very underutilized diagnostic tool. 
In the early 1900s, dysfunction of the sacroiliac joint was a common diagnosis associated with low back and sciatic nerve pain. However, research by Danforth and Wilson in 1925 concluded that the sacroiliac joint could not cause sciatic nerve pain because the joint does not have a canal in which the nerves can be entrapped against the joint. The biomechanical relationship between the sacroiliac joint, the piriformis muscle (see “piriformis syndrome”), and the sciatic nerve had not yet been discovered.
In 1934, the work of Mixter and Barr shifted all emphasis in research and treatment from the sacroiliac to the herniated intervertebral disc, namely lumbar discs. Medical focus on herniated discs was further forwarded by the invention of the MRI in 1977. Recent medical research has revealed that patients with degenerating or herniated discs shown on MRI can frequently complain of little to no pain symptoms, while patients with an unremarkable MRI and no disc pathology can have severe back or sciatic pain. Over-diagnosis and attention on herniated discs has led to the SI joint becoming an underappreciated pain generator in an estimated 15% to 25% of patients with axial low back pain.
The ligaments in the sacroiliac are among the strongest in the body and are not suspected by many clinicians to ever be susceptible to spraining or tearing. Skepticism of the existence of sacroiliac joint dysfunction within the medical community is furthered by the debate on how little or much the sacroiliac joint moves. A discrepancy as large as 2-17 degrees has been reported in clinical findings.
The sacroiliac joint has been found to be the primary nociceptive structure in up to 30% of patients with chronic lower back pain.
Treatment is often dependent on the duration and severity of the pain and dysfunction. In the acute phase (first 1–2 weeks) for a mild sprain of the sacroiliac, it is typical for the patient to be prescribed rest, ice/heat, physical therapy, and anti-inflammatory medicine.
If the pain does not resolve in the first 1–2 weeks, then the patient may benefit from a steroid and anesthetic mixture fluoroscopically injected into the joint (this also serves in confirming diagnosis), as well as manipulative or manual therapy. For the most severe and chronic forms of sacroiliac dysfunction, treatment should proceed with the support of a sacroiliac belt, a series of prolotherapy injections to aid in regeneration and healing of the surrounding injured ligaments, injection therapy, and finally, surgery. The anti-inflammatory effect of injection therapy is not permanent, and the injections do not offer an opportunity to stabilize an incompetent joint. Surgery is often considered a last resort, but for some patients, it is the only method of effectively stabilizing the loose joint. A fixation of the joint (screws or similar hardware only, without the use of bone grafting) is more common than a spinal fusion, as it is much less invasive, surgically straightforward, and results in a quicker recovery time for the patient. Some experts in the field believe that it is important to make sure the sacroiliac joint is in an anatomically correct position prior to fixation or fusion, but published research contradicts this belief. The problem of alignment of the pelvis is that this does not accurately indicate the true status of the inside articular structure; the SIJ.