The Sacroiliac Joint and Its Ligaments

The sacroiliac joint connects the sacrum — the wedge of fused vertebrae at the base of the spine — to the ilium on each side. It is the only structural link between the spine and the legs, so every step, every stair and every lift passes load through it.

It is a hybrid joint. The lower, front portion is a true synovial joint with cartilage-covered surfaces. The upper, rear portion is not a joint surface at all but a syndesmosis: a dense mass of interosseous ligament packing the space between the two bones. That is unusual, and it is the single anatomical fact that most changes how the joint is assessed and treated.

Movement is small. The joint is not designed to glide; it is designed to transfer force while deforming very little. This is why the popular idea of a sacroiliac joint that slips out of place and needs putting back is not supported by what is measurable at the joint.

Two mechanisms hold the pelvis together under load, described by Vleeming and colleagues as form closure and force closure.

Form closure is the stability that comes from the shape of the parts. The sacrum is wedged between the ilia, the joint surfaces carry matching ridges and grooves that produce a high coefficient of friction, and the binding ligaments are among the strongest in the body. A perfectly form-closed joint would need no muscular help at all — but it would also be immobile.

Force closure is the extra compression supplied by ligaments, fascia and muscle acting across the joint, plus the ground reaction force. It is what allows a joint with imperfect form closure to still tolerate a vertical load. Force closure depends on sacral nutation — a small forward rotation of the sacrum relative to the ilia — which tightens the dorsal ligaments and presses the posterior ilia together in anticipation of loading.

This is a model, not a measurement. It is useful because it predicts why some people improve with better muscular control and a period of load management, and why others do not. It is not a licence to tell someone their pelvis is unstable — see lumbopelvic instability for where that idea holds up and where it does not.

This is the finding that reorganises everything. The 2025 multispecialty consensus guidelines concluded that the prevalence of intra-articular and extra-articular pathology in SIJ complex pain is comparable — pain arises from the ligamentous and posterior structures about as often as it arises from inside the joint.

The guidelines went further. They found the evidence slightly stronger for extra-articular corticosteroid injection than for intra-articular injection in providing short-term relief, and they concluded that intra-articular injections have diagnostic validity for intra-articular pain but not for extra-articular pain. One participating society abstained on the extra-articular statement; the rest reached consensus.

The practical consequence: an injection into the joint cavity that does not relieve the pain has not ruled out the SIJ complex as the source. It has ruled out one half of it. That is a distinction the older literature — and much of what patients read online — does not make.

The posterior SIJ complex is supplied by the lateral branches of the sacral dorsal rami, usually S1 to S3 and often with a contribution from the L5 dorsal ramus. These branches form a variable network across the back of the sacrum — a posterior sacral network with no reliable, repeatable topography from person to person.

That variability is the reason nerve-targeted procedures at this joint are technically demanding and why the guideline evidence favours larger, more aggressive lesions over precise small ones. A cadaveric study by Roberts and colleagues found that needles placed along the lateral sacral crest under ultrasound would capture on average 93 per cent of the posterior sacral network if a strip lesion were created between them — an argument for the strip approach rather than for point targeting.

The front of the joint has a separate and even less consistent supply from the ventral rami. No current procedure reliably denervates it, which is part of why nerve ablation at this joint is described as a treatment for extra-articular rather than intra-articular pain.

It rules out the idea that the sacroiliac joint moves in and out of position. Measured motion at the joint is very small, and no palpation test has been shown to detect it reliably — a point developed on the diagnosis page.

It rules out the idea that a single injection anywhere near the joint treats the whole complex. The joint cavity and the interosseous ligament are separate compartments requiring separate approaches.

And it rules out the idea that the pelvis can be assessed in isolation. The long dorsal ligament changes tension when the latissimus dorsi, erector spinae, gluteus maximus or sacrotuberous ligament are loaded. Anything that alters hip, trunk or hamstring function alters what this joint experiences.