The sacroiliac joint (SI joint) is located in the pelvis and links the iliac bones (pelvis) to the sacrum (lowest part of the spine above the tailbone). The SI joint is essential for shock absorption to prevent impact forces from reaching the spine while walking.
The sacroiliac joint is stabilized by a system of ligaments and muscles, which also limit motion. The normal sacroiliac joint has a small amount of normal motion of approximately 2-4 mm of movement in any direction.
SI joint pain can result from conditions such as degeneration, disruption, tumor, infections, inflammatory conditions such as ankylosing spondylitis, and other problems. When the SI joint is injured people can feel pain in their buttock and sometimes lower back; this is especially true when lifting, running, walking or even sleeping.
It is common for pain from the SI joint to mimic disc or low back pain. Publications and clinical research show the SI joint as a pain generator in up to 25 percent of low back pain and up to 43 percent of post-lumbar and lumbosacral fusion patients (Sembrano et al, Spine 2009; DePalma et al, Pain Medicine 2011).
Physical examination with provocative tests along with diagnostic imaging is performed to diagnose and differentiate between low back and the SI joint pain. In addition, the most relied upon method to determine whether the SI joint is the cause of low back pain symptoms is to inject the SI joint with a local anesthetic. The injection is delivered with either fluoroscopic or CT guidance to verify accurate placement of the needle in the SI joint. If symptoms are decreased by at least 75 percent, it can be concluded that the SI joint is either the source of, or a major contributor to lower back pain. If the level of pain does not change after SI joint injection, it is less likely that the SI joint is the cause of low back pain.