Anterior (Forward) Anterior dislocations, where the ball or top of the humerus dislocates forward is by far the most common form of instability or direction of the dislocation. Next is posterior where the ball of the shoulder goes back and then there are shoulders that can dislocate in many different directions... Or multi-directional instability.
Anterior. Usually they dislocate anteriorly and inferiorly, meaning the arm bone dislocates forward and down.
Good Question. As we have learned more about shoulder instability with greater experience and skill in shoulder arthroscopy, it has become increasingly clear that the shoulder is a circle and it is important to treat anatomic pathology to rebalance the shoulder globally. Also, keep in mind that many posterior injuries are missed which may explain why posterior wear is most common pattern in arthritic shoulders.