The acromioclavicular joint (ac joint) is a joint that connects the clavicle and scapula. This joint is held together by the acromioclavicular ligaments and the coracoclavicular ligaments.
The ac joint can be injured from a direct blow to the top of the shoulder. This will typically occur as the result of a fall, often in connection with some sports related activity, which causes a disruption of the ac joint ligaments. The hallmark clinical sign of an ac separation (separated shoulder) is a deformity at the superior aspect of the shoulder. This deformity is a result of the the scapula sagging due to lost connection to the clavicle.