The AC (acromioclavicular) joint is a joint in the shoulder where the collarbone (clavicle) meets the shoulder blade (scapula). The specific part of the scapula adjacent to the clavicle is called the acromion, hence the name AC joint. The AC joint serves as a primary link between the axial skeleton and the upper extremity. The joint has dynamic and static stabilisers, and it is movable in all planes, so it is not a rigid structure. Its complex ligamentous structure is critical to the normal function of the shoulder girdle. The acromioclavicular and coracoclavicular ligaments are the static stabilisers, whereas the deltoid and trapezoid muscles are dynamic stabilisers.

An acromioclavicular joint injury, commonly known as a shoulder separation, is a major injury to the acromioclavicular (AC) joint with disruption of the acromioclavicular ligaments and/or coraco-clavicular (CC) ligaments. These injuries are commonly seen in road traffic accidents involving falls from a bike directly on the shoulder.