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. Diagnosis
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 imaging test SPECT/CT which can sometimes 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.
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 people 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 natures 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.In some people, the sacroiliac joints reverse the normal concave-convex locking relationship, which can lead to rotational misalignment.