Causes

The shoulder joint is the most frequently dislocated joint of the body. Because it moves in several directions, your shoulder can dislocate forward, backward or downward, completely or partially, though most dislocations occur through the front of the shoulder. In addition, fibrous tissue that joins the bones of your shoulder can be stretched or torn, often complicating the dislocation.

It takes a strong force, such as a sudden blow to your shoulder, to pull the bones out of place. Extreme rotation of your shoulder joint can pop the ball of your upper arm bone out of your shoulder socket. Partial dislocation — in which your upper arm bone is partially in and partially out of your shoulder socket — also may occur.

A dislocated shoulder may be caused by:

• Sports injuries. Shoulder dislocation is a common injury in contact sports, such as football and hockey, and in sports that may involve falls, such as downhill skiing, gymnastics and volleyball.
• Trauma not related to sports. A hard blow to your shoulder during a motor vehicle accident is a common source of dislocation.
• Falls. You may dislocate your shoulder during a fall, such as from a ladder or from tripping on a loose rug.