The term sacrum means tailbone. The term ilium refers to the hipbone. Therefore, this joint is known as the tailbone/hipbone joint. As a medical piece of trivia and fact, the ligaments that hold the sacroiliac joint together are the strongest in the human body. At a certain time in life, namely pregnancy, the ligaments of the SI joint becomes the weakest of the human body to accommodate the descent of the fetus. Therefore, this type of injury is more common in women.
The ligaments that hold the SI joint together can be sprained by a stretching out mechanism of injury just as an ankle can be sprained stretching the distance between the ankle bone and the foot bone on the out side of the ankle joint.
Most often the sacroiliac (SI) joint injuries are not revealed on a plain x-ray or an MRI. A diagnostic and potentially therapeutic test would involve the injection of a corticosteroid agent and anesthetic agent under fluoroscopy (x-ray guidance) into the joint to evaluate for pain relief.
It is also often underappreciated that the SI joint, like many other joints in the human body, can refer pain elsewhere. Most commonly, the SI joint refers pain in to the buttock and posterior (back) thigh. There are provocative clinical examination tests also to help confirm this diagnosis.