An intra-articular fracture-dislocation of the acromioclavicular joint.

A hybrid acromioclavicular joint fracture-dislocation in which an intra-articular fracture of the distal clavicle coexisted with a coracoclavicular ligament injury is described. The proposed mechanism of injury is a fall on the point of the shoulder that simultaneously drives the scapula both anteriorly and inferiorly. Forced anterior scapular protraction caused a displaced horizontal plane fracture of the inferior half of the distal clavicle, which was separated from the remainder of the clavicle. The inferiorly acromioclavicular ligament was still attached to this distal clavicle fragment. The inferiorly directed force ruptured the coracoclavicular and superior acromioclavicular ligaments.