The SI joint is a connection at the bottom of the spine between the tailbone (sacrum) and both pelvises (ilia). The SI joint or sacroiliac joint is a rigid joint that transfers forces from the trunk to the pelvis and legs. The range of motion within this joint is minimal. With trauma, the pain localizes just below the lower back and can radiate to both buttocks.
Research shows that the SI joint is involved in about 15 to 35 percent of people with low back pain. By performing the right tests, it is easy to determine whether the pain, or part of the pain, comes from the SI joint. It is unclear what exactly causes the SI joint to become painful. Because many forces are exerted on the SI joint, it is sensitive to overload. It is expected that pain of the SI joint is often the result of improper functioning of the low back and/or the hip(s).