Your shoulder joint is made up of three bones: your collarbone, your shoulder blade, and your upper arm bone. The top of your upper arm bone is shaped like a ball. This ball fits into a cuplike socket in your shoulder blade. A shoulder dislocation is an injury that happens when the ball pops out of your socket. A dislocation may be partial, where the ball is only partially out of the socket. It can also be a full dislocation, where the ball is completely out of the socket.