The sacroiliac joint (SI joint) is a joint that links the pelvis and lower spine. It is made up of the bony structure above the tailbone, known as the sacrum, and the top part of the pelvis, known as the ilium. The SI joint is a true diarthrodial joint that joins the sacrum to the pelvis. It is a synovial joint surrounded by a fibrous capsule containing a joint space filled with synovial fluid between the articular surfaces. The joint surface is smooth in juveniles and becomes irregular over time. The SI joint is highly dependent on its strong ligamentous structure for support and stability. The most commonly disrupted and/or torn ligaments are the iliolumbar ligament and the posterior sacroiliac ligament. The sacroiliac joint is a pain-sensitive structure richly innervated by a combination of unmyelinated free nerve endings and can be a source of pain and inflammation if the SI joint is dysfunctional. Symptoms of sacroiliac joint dysfunction include pain in the buttocks and lower back, which can also affect the legs, groin, and even the feet.