Back pain is a common complaint. Most often the cause is something like a pulled muscle or overstretched ligament, and it is short-lived. Deterioration of cartilage and disks in the spine can also lead to back problems. For a smaller number of people with back pain, the cause is a type of arthritis called ankylosing spondylitis (AS). This type of arthritis causes inflammation at the joints in the spine and the joint at the bottom of the spine that connects it to the pelvis (sacroiliac joint), resulting in pain and discomfort.