The shoulder is a ball and socket joint. Normally the ball (humeral head) sits inside the socket (glenoid). A shoulder dislocation occurs when the ball is no longer sitting inside the socket. Most shoulder dislocations are anterior dislocations, when the ball pops forward and sits in front of the socket. Posterior shoulder dislocations are much less common but more commonly occur after seizures or electrocutions. Some shoulders can dislocated in multiple directions (multi-directional instability), and this usually occurs in “double-jointed” (aka ligamentously lax) individuals.
When a shoulder dislocates, it usually damages some structures in the shoulder joint. In the younger patient, the most common structure to be damaged is the glenoid labrum, which is a piece of cartilage on the rim of the socket which serves to make the socket deeper and therefore the shoulder more stable. If this labrum gets stripped off the socket (aka Bankart lesion) then the socket is now more shallow, and the shoulder is therefore more prone to recurrent dislocations in the future.
If a shoulder is dislocating recurrently, then this problem can be fixed by re-attaching the labrum back to the socket to make the socket deep again ( aka Bankart repair or shoulder reconstruction). This is usually done by using specially designed small anchors which go into the edge of the socket, and which have stitches attached which are used to grab and secure the torn cartilage rim back to the socket. The whole procedure can be done either as a traditional “open” operation, or using newer arthroscopic techniques.
Every time a shoulder dislocates a little more damage is done to the shoulder. Sometimes, bone is worn away either from the socket (bony Bankart lesion) or the ball (Hill Sachs Lesion). When the amount of bone that has been worn away is large, then sometimes in order to make the shoulder stable the bone must be replaced. There are a number of ways that this can be done, with one of the more common operations involving taking a piece of bone from the shoulder (coracoid) and transferring it to the socket (aka Laterjet procedure).
In the older patient with a shoulder dislocation, sometimes the muscles can be torn off. This is known as a rotator cuff tear. When this happens, the tear can be very large and the patient can lose the ability to raise their arm above their head. This is generally considered a situation that must be addressed in the short term by not only putting the shoulder back into place, but by also repairing the torn tendons.