What is a shoulder dislocation and what is a recurrent shoulder dislocation? The shoulder is a ball and socket joint in which the humerus head is the ball and the glenoid cavity is the socket. The glenoid cavity is reinforced by a thickened layer of capsule called the labrum, which causes the deepening of the glenoid socket. In extremes of shoulder position or movement, abnormal forces will cause the humerus head to slip out of the cavity which is called a shoulder dislocation. If the shoulder dislocation happens more than one time, it is called recurrent shoulder dislocation