Dislocations of the shoulder joint are common in sports involving contact elements such as Rugby and American football. They also occur frequently from falls, usually onto an outstretched arm. The most common position for a shoulder dislocation is with the arm out to the side and rotated outwards. This results in the most common form of shoulder dislocations, an anterior dislocation, which make up to 95% of all dislocations. This is where the head of the humerus (top of the arm bone) moves forwards of the shoulder joint. Dislocations can also be posterior, inferior, superior or intra-throacic.
The reason shoulder dislocations are so common is the level of mobility and hence instability, of the shoulder joint as a whole. The shoulder has the largest range of motion of any joint in the body, but the congruency of the articulating bones (humerus and scapula) is very poor. The joint therefore relies on the support of the surrounding soft tissues – the joint capsule, labrum, ligaments and muscles.
Shoulder dislocations are often considered to be fairly minor injuries, however the damage that can be associated with a dislocation can be very severe. The most common damage is to the glenoid labrum. This is a ring of cartilage surrounding the socket of the joint (part of the scapula or shoulder blade) which acts to increase the congruency of the joint by effectively deepening the socket. Damage to other soft tissues such as rotator cuff tendons, nerves and blood vessels are also common.
For this reason it is important that a dislocated shoulder is never put back in place by someone who is not trained to do so. Usually, an X-ray or MRI will be taken before the shoulder is reduced to check for any associated damage. After this, depending on the direction of dislocation, a manoeuvre will be performed to allow the humerus to move back into position. The arm will then be immobilised in a sling for a week or more.
After a dislocation the chances of the same shoulder dislocating again are higher. This is due to laxity in the joint capsule and ligaments surrounding the joint after they were stretched during the first injury. Any associated injuries also make repeat dislocations more likely.
In order to prevent recurrent dislocations, a period of extensive rehabilitation is required. This should aim to strengthen the muscles surrounding the shoulder joint, improve proprioception (the sense of the joints position), and restore full range of motion.