The AC joint is where the acromion (a part of the shoulder blade) connects to the clavicle (collar bone) at the top of the shoulder. This joint helps maintain the position of the shoulder and is very important for shoulder control, motion, and strength. The joint is stabilized by a capsule and ligaments, and injury occurs when these structures are damaged. When the capsule and ligaments are injured, the joint can become unstable and painful, and shoulder function can be affected. This type of injury is referred to as an “AC joint sprain” or a “separation.” These injuries typically occur after a sudden fall onto the shoulder. The AC joint is also a common place that arthritis develops, which can become painful. This is usually the result of years of degeneration or “wear and tear” of the joint and is common in weight lifters and laborers.