Sacroiliac joint dysfunction is a term used to describe pain in the sacroiliac joint. The sacrum sits at the lower end of the spine, just below the lumbar spine. The sacroiliac joint sits between the sacrum and the iliac bone and is one of the larger joints in the body. Often, an exact cause of the painful sacroiliac joint cannot be found, although injury is thought to be a common cause (such as injury during a motor vehicle accident).
The most common symptom of SI joint dysfunction is sacroiliac joint pain. Patients often experience pain in the lower back or the back of the hips. Pain may also be present in the groin and thighs. In many cases, it can be difficult to determine the exact source of the pain.
Catherine Burt Driver, M.D., Sacroiliac Joint Dysfunction (SI Joint Pain), Medicine Net, https://www.medicinenet.com/sacroiliac_joint_pain/article.htm#what_are_the_sacroiliac_si_joints