Approximately 22 percent of low back pain is actually caused by the sacroiliac joint which connects your pelvic bones to the sacrum or lowest part of your spine above the tail bone. Like any joint, the SI joint can be prone to degeneration over time with wear and tear or could be impacted by a fall or some other trauma to the pelvis. The pain in the low back or buttocks caused by this joint is felt especially when lifting, running or walking, or when lying on the affected side. The pain is sometimes very similar to that caused by a disc in the lower spine. The good news is that if the SI joint is determined to be the cause of the pain, a new minimally invasive surgery can be used to fuse the joint through a tiny incision in your hip. They key is proper diagnosis with x-rays and CT scans. In some cases, a physician may inject painkillers directly into the joint, if there is relief of the pain from this procedure, then the SI joint is the likely cause.