What happens when a shoulder dislocates
The shoulder is the most mobile joint in the body, and that is precisely its weak point. The head of the humerus rests on a small, shallow surface, the glenoid — the usual comparison is a golf ball on a tee — and what holds it there is not bone but soft tissue: the labrum, the capsule, the ligaments and the rotator cuff muscles.
In a dislocation, the head of the humerus comes off that surface, almost always forwards and downwards. In doing so it drags the labrum and tears it off the bony rim. Even once the shoulder is put back, that stop remains detached, and this is where the real problem begins: the shoulder has lost part of its natural brake.
Why it recurs
This is the question that worries people most, and the answer depends above all on the age at which it first happened. In a young person the tissue is elastic and the labral injury does not heal in the right position, so the stop is not restored; on top of that, they usually play sport and will expose the shoulder to the same situation again. In an older person the tissue is stiffer, heals with more fibrosis and the functional demand is lower.
Each new dislocation makes things slightly worse: it wears down the bony rim of the glenoid and produces a notch in the head of the humerus. Once enough bone is lost, repairing the labrum alone is no longer sufficient and a technique providing a bone stop is required. This is why it is not always wise to wait and accumulate episodes.
Instability without dislocation
Not every case has fully come out. Some shoulders have never truly dislocated but give way, slip or feel unsafe in certain positions. Sometimes this is the consequence of an old episode, and sometimes it is constitutional laxity: people with more elastic joints than usual, in whom the shoulder subluxes in several directions.
The distinction matters a great deal, because the treatment is almost the opposite. Post-traumatic instability with a labral injury usually ends in theatre if it is limiting. Multidirectional laxity responds very well to rehabilitation and responds badly to surgery, so operating on it is normally a mistake.
