The pain at the junction of the waist and buttocks refers to the pain in the sacroiliac joint, which usually has three causes: 1, because there are many muscles, tendons, ligaments and other soft tissues attached to the sacroiliac joint, if you sit for a long time, squatting for a long time and bending hard to lift heavy objects, it may lead to the formation of local strain and injury, so that there will be a sterile inflammatory lesions, inflammatory metabolites will stimulate the surrounding peripheral nerves. 2, ankylosing spondylitis, which is an autoimmune disease, is due to the immune system of their own immune response to their own tissues, resulting in degeneration of the sacroiliac joint cartilage, worm-like changes, the formation of some bone hyperplasia and calcification, with the development of the disease, may extend up the direction of the spine, there will be a significant pain. 3, sacral Iliac arthritis, such as sacroiliac joint injury, infection and excessive activity caused by strain, may lead to cartilage degeneration, osteophytes and calcification gradually formed, so that the formation of inflammatory lesions, the peripheral nerves around the stimulation, there will also be obvious pain.