The shoulder bones are held together by a group of muscles that are called the rotator cuff muscles. These muscles are responsible for the shoulder's fine movements, such as throwing a ball.

Because of the shoulder's shallow socket and lack of ligaments, any weakness of the small rotator cuff muscles makes it easy for the head of the shoulder to slide part way out of the socket, which is a partial dislocation, or subluxation. Or it may slide all the way out, which is a full dislocation.

The...