What is the Sacroiliac Joint?

Mary McMahon
Mary McMahon

The sacroiliac joint is the joint which connects the spine to the pelvis. This key joint in the body actually comes in the form of a pair of joints which function together, the left and right sacroiliac joints. Sometimes called the SI joint, this joint can be subject to a variety of medical issues which can cause discomfort, pain, and difficulty walking in people of all ages. Older people tend to be more at risk of SI joint dysfunction.

An X-ray of the pelvis, showing the sacroiliac joint.
An X-ray of the pelvis, showing the sacroiliac joint.

This joint connects the sacrum at the end of the spine with the pelvis. Small dimples at the base of the lower back correspond with the location of the sacroiliac joint. The joint serves several different functions in the body. One function is to act as a shock absorber for the spine, distributing the shock of walking across the pelvis to reduce the strain on the spine. This joint also stabilizes the body during walking, working in concert with the pelvis so that people can walk and run upright.

Inflammation of the sacroiliac joint can cause lower back pain.
Inflammation of the sacroiliac joint can cause lower back pain.

Another function of this joint is to convert and carry twisting movements. Torsion would cause the pelvis to crack and split, which is not desired, and this joint is designed to twist while keeping the pelvis stable and intact. At the area of the joint, the bones have rough surfaces which interlock, and the joint is stabilized and supported by a network of ligaments which are designed to keep the joint in place.

Sacroiliitis is an inflammation of the sacroiliac joint. Sacroiliac inflammation can happen for a number of reasons including physical strain, infections, spinal arthritis, and so forth. Patients with this condition feel pain and stiffness around the pelvis and lower back. SI joint dysfunction, a related condition, occurs sometimes during pregnancy as a result of hormones which soften the joint, and can occur as a result of damage to the ligaments as well. In both cases, patients may find it difficult to move, and may experience pain with movement of the pelvis.

When joint pain and other problems are identified in the sacroiliac joint, medical imaging studies and other diagnostic tests may be used to learn more about the cause of the pain. These studies provide information about the site of the dysfunction so that a doctor can determine which treatments might be most appropriate. Treatments can include medications, surgery, physical therapy, massage, and a variety of other tools which are intended to restore comfort and a free range of motion to the joint.

Physical therapy is an important step in treating sacroiliitis.
Physical therapy is an important step in treating sacroiliitis.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


@feruze-- No, pregnancy isn't the only way you can develop SI joint dysfunction. There is actually several different factors that can affect your sacroiliac joints.

Did you fall or have any kind of accident before the pain started? A sacroiliac joint injury can easily cause dysfunction. And has your doctor checked for degenerative diseases like arthritis or rheumatism? Or were you checked for an infection?

All of these can lead to problems with the sacroiliac joint. Blood and urine tests, and an MRI can identify the cause. So I highly suggest you get these done if you haven't yet.


@feruze-- Hi! I understand what you're going through because I was also confused with these terms for a while. The confusion stems from the fact that doctors often use these two terms interchangeably. And they are definitely closely linked to one another but they are not the same thing.

SI joint dysfunction is exactly what the name suggests. It's when the sacroiliac joint doesn't work properly. It doesn't move in the way it should be.

Sacroiliitis (this is what I have) is an inflammation of the sacroiliac joint. This doesn't mean that there is a problem with how the joint moves. The reason that these are usually used together is because SI joint dysfunction usually leads to sacroiliitis. And the inflammation can combine symptoms of sacroiliitis together with the joint dysfunction.

So it's very much possible that you have both of these right now. Ask your doctor to clarify if you're not sure.


Can sacroiliac joint dysfunction happen to women who have not given birth? And what differentiates SI joint dysfunction from sacroiliitis? Aren't they both caused by inflammation of the sacroiliac joint?

I was diagnosed with SI joint dysfunction six months ago. I have been going to physical therapy since then. I also wear a special belt during the day. This diagnosis was made after I had a month of pain in my lower back and leg. As far as I understand, the dysfunction is caused by my sacroiliac joint not staying in place. My physical therapist always tells me that he is re-aligning my lower back.

What I don't understand however, is why I have this condition in the first place. I have never been pregnant, nor do I engage in strenuous sports. The pain came about suddenly and I have been miserable since.

Is anyone else suffering from this horrible condition?

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