The Sacroiliac (SI) Joint is where your hips meet the spine. If these joints become damaged from injury or diseased from arthritis, it can cause pain. Pain in the SI can also be caused if the supporting ligaments become irritated. The pain can start in the lower back and radiate down into the leg when walking or just sitting. Treatment, depending on the cause, can consist of physical therapy, or injections of medicine into the SI joint.