How the SI joint works

The SI joint is a shock absorber for your pelvis and spine.

Your two SI (sacroiliac) joints, one on each side of your pelvis, connect the bone at the bottom of your spine (the sacrum) with the bones on the side of your pelvis (the ilia). They work like other joints – they have a covering of cartilage, some fluid, and a capsule around them to keep everything in place. They also have lots of nerve endings. When you bend, walk, and twist, the SI joints move just a little bit (2 – 4 millimeters) to absorb the impact from jumping, stepping, or landing.

When things go wrong, it can really hurt.

Because the SI joints have lots of nerve endings, if they get inflamed or damaged, they can cause a lot of pain. And, unfortunately, the pain doesn’t just affect your pelvis or hips; instead, it often radiates out into your low back, your butt, your legs, or even all three.

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How the SI joint gets hurt

There are lots of ways for the SI joint to get inflamed or injured, even if it’s just through wear and tear.

You might have experienced or been diagnosed with one of the common causes of SI joint pain, but it’s also possible to have SI joint pain without a clear cause.


Sacroiliitis is really just a way of saying that the SI joints are inflamed. It’s helpful only because it indicates there’s no specific injury like one of the causes listed below. When you have sacroiliitis, SI joint pain often extends down the legs or into the butt and gets worse when you stand for a long time.


This is what most people mean when they say “arthritis,” and it happens when the cartilage that protects your joints gets worn away so that the bones rub together and cause pain. When that happens to the SI joint, it usually causes low back pain.


Any time you experience an injury to the back or pelvis, your SI joint might be affected. Some of the most common causes of this kind of injury are falls, car accidents, and sports injuries.

Walking pattern

Anything that makes you walk differently than normal can put stress on your SI joints and may eventually cause SI joint problems. Possible causes of an altered gait might be things like ankle or knee injuries, but could also include having one leg longer than the other, or having a condition like scoliosis (too much curve in your spine).


It’s possible for the SI joint to get infected, but it’s pretty rare. If it happens, it usually means you have a fever and often intense pain in the thighs when walking. If you think this is the case, please see a doctor right away.

Lumbar fusion surgery

A lot of people with SI joint pain might already have had a lumbar fusion (back surgery to fuse two vertebrae together). Since this kind of spine surgery changes how your spine distributes weight, it can cause SI joint stress – or it’s possible that some of the back pain you were hoping to fix with the lumbar fusion is actually SI joint pain. SI joint dysfunction often coexists with lumbar problems. In fact, previous lumbar surgery sometimes contributes to SI joint dysfunction.* *DePalma, ey al. Etiology Chronic Low Back Pain in Patients Having Undergone Lumbar Fusion Pain Medicine 2011; 12:732 - 739


Pregnancy makes the ligaments in your pelvis stretch out, which means the SI joints can move more and can cause pain. For most women, the ligaments go back to normal afterward, but for some women, they remain loose, which can mean continued pain. Pregnancy can also make you walk differently, which can make the problem worse.

How you can tell it’s your SI joint

Back pain can be hard to diagnose, but SI joint pain has a few tell-tale signs.

Low back pain

This is the big one, and sometimes can be the only sign of SI joint pain. Low back pain means pain that you feel pretty far down on your back – around where you wear a belt. It can feel different for different people, and, especially when it’s been going on for a long time, can be difficult to describe – it may be aching, stabbing, or dull.

Butt pain

A literal pain in the butt that usually feels like a dull ache on one side and might extend down into your leg or even into your groin. This kind of pain is often blamed on sciatica, which certainly can be a cause. But so can SI joint pain.

Pain when you climb stairs

If your back hurts when you’re climbing stairs, it might be because the twisting motion stairs cause in your hips is moving inflamed or injured SI joints. Climbing hills or vacuuming might also cause this kind of pain.

Pain when you sit

If you have back pain when you sit, especially on one side or the other, that makes you feel like you need to keep shifting your weight back and forth, it might be a problem with your SI joint. You might also have pain when you lie down to sleep.

Whatever pain you’ve got, you deserve relief.
If you’ve got some of these symptoms, your SI joint might be to blame. But whatever’s going on, remember that low back pain isn’t something that you should have to live with.

Find out more about diagnosis and treatment.

Although SI joint fusion surgery isn’t right for everyone, it can benefit those patients who are appropriate candidates. As with any type of surgical procedure, there are risks associated with SI joint fusion surgery. Be sure to discuss these with a spine specialist who is skilled in diagnosing and treating patients with SI joint dysfunction.