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    You may not have ever known you had a sacroiliac joint, much less where it is. Until it starts to ache. Then, it’s hard to miss how much you use it to move around and do your everyday activities.

    You have two sacroiliac joints. They connect your pelvis with the lower part of your spine. Most joints, like the knee and hip, have a wide range of movement, but the sacroiliac rotates and tilts only slightly. That lets it do its main job: to stabilize and support your pelvis, help to transmit the weight of your upper body to your legs, and act as a "shock absorber" when you walk or run.

    The sacroiliac joint can be injured in a number of ways, making even daily activities, like sitting, walking or climbing stairs, difficult. This is called sacroiliac joint dysfunction. 

    Causes

    The most common cause of SI joint dysfunction is injury from a car accident or fall.  But it can also happen from:

    • Sports injuries such as a football tackle
    • Stress or injury to the joint over and over, such as from jogging for many years
    • Older age
    • One leg that’s shorter than the other
    • A spinal injury
    • Scoliosis (abnormal curvature of the spine)
    • Spinal surgery, especially operations that fuse the lower part of the spine, called the sacrum
    • Pregnancy. The hormones that a woman’s body makes near the time of delivery can cause the pelvis to relax and change position. Weight gain, changes in posture, and the childbirth process can also cause problems in the joint.

    Half of people with SI joint pain can trace it back to a specific event, like that car accident or sports injury. Other times, though, there's no obvious reason for the problem.

    Symptoms

    Pain is often the main symptom, usually in the lower back and buttock and sometimes the back and upper leg.

    Some people also feel SI joint pain in the groin, belly, and even their feet. Studies have found that intense pain in the central area of your body is more likely to spread to the outer parts.

    Aches from SI joint dysfunction usually show up on one side of the body rather than both sides.

    Low back and leg pain have many possible causes, so you’ll need to work with your doctor to figure out if your SI joint is the reason you hurt.

    WebMD Medical Reference