The acromioclavicular joint (also called the AC joint) is the area where the clavicle meets the highest point of the shoulder blade. An injury to this structure that is common is an AC separation, where a force causes the ligaments that attach to the underside of the clavicle to tear.
What causes an AC separation?
The most common reason for a person to suffer an AC separation is a fall directly onto the shoulder. The force causes the ligaments to be injured and the collarbone separates from the wing bone (shoulder blade). This type of injury will cause the wing bone to move downward with the weight of the arm, creating a bulge above the shoulder.