There are many reasons for the pain behind the left buttock bone, but the most common ones are caused by the following conditions: a. If the patient has local osteoarthritis or femoral head necrosis, it will cause painful symptoms in the left buttock bone. For early stage femoral head necrosis, a local MRI is needed to make a clear diagnosis, while for late stage femoral head necrosis, an ordinary X-ray is usually taken to clarify. Second, if the patient has localized sacroiliac arthritis or ankylosing spondylitis, it can also cause painful symptoms in the bone behind the left buttock. Third, if a patient suffers from local trauma causing periosteal injury or fracture, it can also cause pain in the back bone of the left buttock. For such patients, a general X-ray, local CT and 3D reconstruction CT are usually enough to make a clear diagnosis.