The shoulder is the most mobile joint in the body, it is also the most frequent joint to be dislocated. A first time dislocation usually occurs from a traumatic injury such as a fall, rugby tackle or motorcycle / bicycle accident. The most common dislocation is anterior where the humeral head moves forwards out of the socket and towards the chest. A posterior dislocation is less common and may result from an epileptic seizure.
At the time of a dislocation the humeral head will tear the rim of labrum cartilage off the front of the glenoid socket (known as a Bankart Lesion) and stretch / partially tear the ligaments and capsule in the front of the joint. It may also break a small piece of bone off the front of the glenoid socket (Bony Bankart) or cause a small impaction fracture in the head of the humerus (Hillsachs Lesion).
These injuries which occur at the time of initial dislocation make the shoulder more unstable and the risk of re-dislocation increases significantly with each subsequent dislocation.