There are two sacroiliac joints. One on each side where the sacrum joins the left and right ilium (hip bones). It acts as shock absorber between the upper and lower body. This joint is like any other joint and cause pain usually because of degeneration and sometimes trauma. Other structures in the spine can cause pain indistinguishable from the SI joint. The best and most reliable way to figure out if the SI joint is a major source of pain is to inject numbing medication in the joint and see if it makes a big difference. Sometimes steroid is included to hopefully produce a long term benefit.