Overview
Sacroiliitis is an inflammation of one or both of the sacroiliac joints, which connect the lower spine and pelvis. It can cause pain in the buttocks or lower back and may even extend down one or both legs. The pain associated with sacroiliitis is often aggregated by prolonged standing or stair climbing.
Causes
Traumatic injury
Arthritis
Pregnancy
Infection (rare)
Symptoms
Pain in the buttocks and/or lower back
Pain in the legs, groin, or feet (less common)
Pain aggravated by prolonged standing, bearing weight more on one leg than the other, stair climbing, running, large strides, extreme postures
Diagnosis
Physical examination and medical history consultation
X-rays
CT scan
MRI
Lab tests (if infection is suspected)
Anesthetic injections
Treatment
OTC pain medication
Non-narcotic pain reliever
Muscle relaxants
Corticosteroids
Disease-modifying antirheumatic drugs (DMARDs)
TNF inhibitors
Physical therapy
Surgery
Source: http://www.mayoclinic.com/health/sacroiliitis/DS00726