Thigh and lumbar joint pain, i.e. sacroiliac pain, common causes are ankylosing spondylitis, sacroiliac joint sprain, lumbar spine disease and so on. 1. Ankylosing spondylitis: Ankylosing spondylitis usually violates the sacroiliac joints first, which is characterized by recurrent lumbar pain, lumbosacral stiffness, lumbar muscle spasm, and pain radiating to the buttocks and lower limbs. 2. Sprain of sacroiliac joint: If the sacroiliac joint is accidentally sprained during exercise or labor, it may also cause sacroiliac joint pain. 3. Lumbar spine diseases: Lumbar spine diseases such as lumbar disc herniation, lumbar spine slippage, lumbar spinal stenosis, etc., will be manifested as pain in the lumbar and sacroiliac areas, which may be accompanied by limited activities and numbness in the lower limbs. Sacroiliac pain may also be caused by other reasons, and should be treated in a timely manner by going to the hospital, completing the examination to clarify the cause of the disease, and then giving targeted treatment or therapy.