There are many parts to the shoulder complex. Most people think of the shoulder as the upper arm bone in a socket. That's technically correct, but the "socket" is just a small part where the arm moves and turns. Above the socket, the collarbone attaches to the shoulder blade as it comes around from behind. A band of strong ligament holds these two bones together. Where these two bones meet is called the acromioclavicular or AC joint.

A shoulder separation occurs when the ligaments at the AC joint are torn or damaged, and these two bones are disrupted. Shoulder separations are graded as I, II, or III depending on the seriousness of the tear. An X-ray determines this. For example, if the ligaments are completely ruptured, the collarbone becomes dislocated. This is a grade III shoulder separation. A grade I is a minor ligament tear, and the bones stay in place. Grades II is in between these two extremes.