Patient Adviser What to Do About AC Joint Injuries

The muscles, joints, and bones of the shoulders form a base of support that allows your arms to swing, lift, or throw (figure 1). One of these bones, the collarbone, is also called the clavicle. Above your arm is an extension of the shoulder blade called the acromion. Where these two bones meet at the top of the shoulder is the acromioclavicular (AC) joint. The AC joint is not the shoulder joint. The shoulder joint is where the bone of the upper arm (humerus) meets a shallow socket that is also part of the shoulder blade.