Sacroiliitis is an inflammation of one or both of your sacroiliac joints — the places where your lower spine and pelvis meet. Sacroiliitis can cause pain in your buttocks or lower back, and may even extend down one or both legs. The pain associated with sacroiliitis is often aggravated by prolonged standing or by stair climbing.

Sacroiliitis occurs when worn-out cartilage in the joints causes sacroiliac joint dysfunction. This type of injury to the spine can be attributed to arthritis, infection, work, over-use, or an accident. Pain may start in the joint, or in surrounding ligaments or nerves. Ligaments are bands of tissue that connect one bone to another. The sacroiliac joint has many nerve endings. The nerves send pain signals to the brain.