The shoulder is the most moveable joint in your body. It helps you to lift your arm, to rotate it, and to reach up over your head. It is able to turn in many directions. This greater range of motion, however, can cause instability.

Shoulder instability occurs when the head of the upper arm bone is forced out of the shoulder socket. This can happen as a result of a sudden injury or from overuse.

Once a shoulder has dislocated, it is vulnerable to repeat episodes. When the shoulder is loose and slips out of place repeatedly, it is called chronic shoulder instability.

Normal shoulder anatomy

Your shoulder is made up of three bones: your upper arm bone (humerus), your shoulder blade (scapula), and your collarbone (clavicle).

The head, or ball, of your upper arm bone fits into a shallow socket in your shoulder blade. This socket is called the glenoid. Strong connective tissue, called the shoulder capsule, is the ligament system of the shoulder and keeps the head of the upper arm bone centered in the glenoid socket. This tissue covers the shoulder joint and attaches the upper end of the arm bone to the shoulder blade.

Your shoulder also relies on strong tendons and muscles to keep your shoulder stable.

Read more at http://orthoinfo.aaos.org/topic.cfm?topic=A00529