Sacroiliac

What it is

The joint where the sacrum meets the ilium on each side, at the back of the pelvis. Its surfaces are irregular and interlocking, and it is bound by some of the strongest ligaments in the body.

It is unusual in being a synovial joint built to resist movement rather than permit it. The irregular surfaces increase friction deliberately, and the surrounding ligamentous structure is extremely robust. In many people the joint surfaces become progressively more fused with age.

What it does

Transfers load between the spine and the legs. Everything your upper body weighs passes through these two joints on its way to your hips.

It moves — a few degrees at most, and measurably less than that in many people. It is one of the least mobile joints in the body that is nonetheless a genuine joint.

The mechanism that keeps it stable is worth knowing: compression across the joint surfaces increases friction and locks it, and that compression is supplied partly by the ligaments and partly by muscles crossing the region, including gluteus maximus and the pelvic floor.

What loads it

Standing, walking, carrying, and every hip hinge. Loading is asymmetrical during single-leg stance, which is most of walking.

What is commonly got wrong about it

This joint attracts more confident and less supportable claims than almost any other.

"SI joint out of alignment", and manual techniques to put it back. The joint moves only a few degrees under very strong ligamentous restraint. Claims to detect subtle positional faults by palpation have been tested repeatedly and inter-examiner agreement is generally poor — different clinicians examining the same person often disagree. Claims to reposition it by hand are not well supported by what is known about the joint's mechanics.

"Pelvic misalignment" and leg-length assessment. Small leg-length differences are extremely common and usually asymptomatic, and the measurement methods used to detect them have limited reliability.

That it is a common source of back pain. It can be a source of pain, and estimates of how often vary widely because the diagnostic tests are imperfect. Back pain is outside this site's scope, and this is a region where confident online explanations should be treated with particular caution.