The extensive range of motion of the glenohumeral joint requires that the glenoid (socket of the shoulder) is shallow. For the shoulder to remain stable, it relies on the labrum cartilage to deepen the glenoid, the ligaments and capsule to offer further stability and for the rotator cuff muscles to centre the humeral head in the glenoid.
When the labrum and cartilage, and to a lesser extent the rotator cuff, are injured, shoulder stability can be reduced.
When a shoulder dislocates, the structures that supported it in place are inevitably injured either in the form of stretching, but more commonly by tearing with or without stretching. It is common for the labrum to become detached from the glenoid (called a Bankart injury) occasionally with a degree of breaking (fracture) of the bone. Unfortunately, unlike some other joints, it is common for the injured structures to fail to heal in their original position, and therefore the shoulder can become unstable (chronic instability). Chronic instability is much more common in patients who dislocate their shoulder when young, especially in the late teenage years and early twenties. It can develop in older people, although it is less common.
If your shoulder is unstable, you may experience symptoms such as your shoulder feeling like it is wobbly, or coming out of the joint throughout certain activities or positions of the shoulder, or it may actually dislocate during some activities. If the instability is minor, it is possible that all you will feel is pain and experience reduced function.