The acromioclavicular, or AC, joint is a joint in the shoulder where two bones meet. One of these bones is the collarbone, or clavicle. The second bone is actually part of the shoulder blade (scapula), which is the big bone behind the shoulder that also forms part of the shoulder joint. The portion of the shoulder blade that meets the clavicle is called the acromion. As a result, where the clavicle meets the acromion is called the AC joint.
Like most joints in the body where bones meet, there is cartilage between the two bones, which is the white tissue between bones that allows them to move on each other, like Teflon on two ball bearings. Arthritis is a condition characterized by loss of cartilage in the joint, which is essentially wear and tear of the smooth cartilage that allows the bones to move smoothly. Like arthritis at other joints in the body, it is characterized by pain and swelling, especially with activity. Over time, the joint can wear out and get larger, with spurs forming around it.
These spurs are a sign of the arthritis and not a cause of the pain. Reaching across the body toward the other arm aggravates arthritis at the AC joint. AC joint wear and tear is common in weight lifters, especially among those who bench press and to a lesser extent, military press. In weight lifters the arthritis at the AC joint has a special name — osteolysis.