Sacroiliac (SI) Joint Dysfunction

What it is

The sacroiliac joints sit where the base of your spine (the sacrum, the triangular bone at the bottom) meets the two wings of your pelvis. There is one on each side, a few inches below the belt line and a couple of inches out from the midline. Every step you take sends the weight of your upper body through these two joints and into your legs.

They are built for load, not for motion. Each joint is shaped like a shallow letter L, with rough, interlocking surfaces, and it is wrapped in some of the strongest ligaments in the body. A healthy SI joint moves only a few degrees, enough to absorb shock and let the pelvis twist a little as you walk.

When one of these joints becomes irritated, inflamed, or starts to move more or less than it should, it hurts. That is SI joint dysfunction. It is a common and often missed cause of low back and buttock pain, and it is responsible for somewhere between one in six and one in four cases of chronic low back pain, depending on the study.

Why it starts

Often there is no single cause. The patterns I see most:

• A fall onto the buttock, a car accident, or a sudden misstep off a curb or a stair. The joint takes a jolt it was not braced for, and the ligaments around it are strained.
• Pregnancy and the months after. Hormones loosen the pelvic ligaments so the baby can pass, the weight shifts forward, and the joints are asked to move in ways they usually do not.
• Uneven loading over years. A leg-length difference, a limp from a bad hip or knee, scoliosis, or a job that keeps you on one leg (ladders, pedals, carrying on one side) all load one joint more than the other.
• After a lumbar fusion. When the lowest part of the spine is fused, the SI joints are the next moving part below it and take on extra work. SI joint pain after fusion is common enough that I check for it whenever back pain returns after a solid fusion.
• Loose joints in general. People with hypermobility, including Ehlers-Danlos syndromes, have SI joints that move more than they should.
• Wear and arthritis in the joint itself, which gets more common with age.

A separate problem, called sacroiliitis, is inflammation of the joint from an inflammatory arthritis such as ankylosing spondylitis or psoriatic arthritis. It tends to show up in young adults, causes morning stiffness that lasts more than half an hour, and improves with movement rather than rest. Blood tests and an MRI help tell it apart, and it is treated with medicines rather than the steps below.

What it feels like

• Pain on one side, low in the back and in the upper buttock. Many people can put one finger on the spot, right over the dimple at the top of the buttock.
• Pain that spreads into the groin, the side of the hip, or the back of the thigh. It can go toward the knee, but it rarely goes past it, and it does not usually cause true numbness or weakness in the foot.
• Pain with transitions: standing up from a chair, getting out of the car, rolling over in bed, climbing stairs, and standing on one leg to dress.
• Pain with sitting on the sore side, or with sitting for a long time on a hard surface.
• A feeling that the pelvis is "out" or unstable, or a catch when you shift your weight.

Because the pain travels, SI joint dysfunction is easy to mistake for sciatica, a hip problem, or a lumbar disc. I see people who have had treatment for the wrong one of those for months.

How we diagnose it

Imaging cannot see SI joint pain. X-rays and MRI of the joint usually look normal, or show the same mild wear that most people have. So the diagnosis rests on three things:

1. Your story. Where the pain is, where it goes, and what sets it off.
2. The exam. I press and stress the joint in several different ways: pushing the knees apart and together, pressing on the sacrum, and placing the leg in a figure-four. When three or more of these maneuvers bring on your familiar pain, the joint is the likely source. I also check the hip and the lumbar spine, because they can hurt in nearly the same place.
3. A diagnostic injection. This is the most reliable test. Under X-ray guidance, a small amount of numbing medicine is placed directly into the joint. If at least three-quarters of your pain disappears for the next hour or two, the joint is the source. If the pain does not change, we look elsewhere. I will not consider a fusion for an SI joint that has not passed this test.

How we treat it

Most people improve without surgery. I work through the steps in this order:

• Physical therapy. This is the foundation. The goal is a stable pelvis: stronger gluteal and deep abdominal muscles that hold the joint steady, looser hip flexors and hamstrings that stop pulling it out of balance, and a correction of any habits that load one side. I have written out a starting program in my home program for SI joint dysfunction. Bring it to your therapist.
• An SI belt. A wide, firm belt worn low around the pelvis compresses the joints and gives many people quick relief, especially during pregnancy or the months after. It is a crutch, not a cure, and it works best alongside the exercises.
• Medicines. An anti-inflammatory for a limited time, with food, if your stomach and kidneys allow it. Acetaminophen for day-to-day control.
• A steroid injection into the joint. This often follows the diagnostic injection or is combined with it. Relief lasts weeks to months and lets you do the therapy properly.

The science: for the curious

Pain from the SI joint travels because the joint shares nerve supply with the buttock and the back of the thigh, so the brain reads an irritated joint as pain in those areas. That overlap is why it mimics sciatica, and why imaging alone cannot settle the question: a herniated disc on an MRI and a painful SI joint can both be present in the same person, and only one of them may be causing the pain. The numbing injection cuts through that uncertainty in a way no scan can. Getting the source right is most of the work.

When to call

Call if the pain is spreading below the knee, if you notice new numbness or weakness in the leg or foot, if you have a fever with back pain, or if the pain is not settling after six weeks of a proper program. New trouble controlling your bladder or bowels is an emergency: go to the nearest emergency room or call 911.

Learn more

OrthoInfo, Low Back Pain: orthoinfo.aaos.org (opens in a new tab)

OrthoInfo, Spinal Injections: orthoinfo.aaos.org (opens in a new tab)

My home program for SI joint dysfunction, and my guides to lumbar radiculopathy and which nerve is pinched: L2 to S1.

This handout is general education about a common condition and does not replace the personalized advice we discuss in clinic. If anything here does not match what we talked about, what we talked about wins: please ask me.