The shoulder joint is a ball and socket joint. Shoulder instability is diagnosed when the ball slips or completely dislocates from the socket.
There are two main types of shoulder instability, both of which can present in a range of ways. One is traumatic instability, which can happen if your shoulder is hit, perhaps in a fall or in an accident. The other is atraumatic instability. This can occur in some people simply because they have "loose joints", or it can develop over time as the result of specific repetitive movements.
If your shoulder joint slips, it is known as subluxation. You may feel the ball slipping, catching or coming out of its joint. This can be associated with pain and a sense of a "dead arm". It may also make you feel apprehensive about moving your arm in certain positions.
If the ball comes completely out of the socket of your shoulder joint, it is called a dislocation. With a dislocation, you may not be able to put it back in to the joint yourself. A dislocation is also more likely to damage the structure of the shoulder.
Your instability may get worse over time. It is possible to get both dislocation and subluxations in the same shoulder. For example, after having dislocated your shoulder in a fall and had it relocated in hospital, you may find the joint slipping when you try a particular activity, such as throwing a ball or swimming.
Why does it happen?
Traumatic instability happens when your shoulder joint collides with a hard object or your arm is forced into a particular position. For example, if you hit it on the ground in a fall, if something bangs into your shoulder in a car accident, or if someone collides with you in a rugby tackle. The ball socket is normally pushed forwards and downwards, usually when your arm is out to the side and twisted backwards. Traumatic dislocations can result in damage to the labrum and recurrent instability.
In a very few instances the ball may be forced out backwards (posterior dislocation). This tends to occur if your arm is in front and across your body on impact. Unfortunately having had this injury once, it tends to recur. Research also shows that the younger you are when you have the first dislocation, the more likely it is to recur.
For some people the shoulder is not involved in a specific accident or event, but the joint gradually feels unstable. This may develop with specific, repetitive movements of the arm (such as throwing balls or swimming). Sometimes people have "loose joints" and these can become a problem and start dislocating or slipping during everyday activities.
The ball may slide forwards (anterior), backwards (posterior) or downwards (inferior). Sometimes it may slide in more than one direction and occur in both shoulders.
A few people can make the muscles pull the ball out of the socket, without moving the arm at all. This is known as "voluntary" dislocation. Often it starts as a party trick but can cause the joint to slip out involuntarily. Do not do this movement! It trains the muscles incorrectly and reminds your brain of an incorrect movement. You will probably always have the ability to do this; however, our advice is not to make use of that ability.
Sometimes it is not clear-cut as to whether you have a purely traumatic or an atraumatic instability problem. The two situations can overlap to some extent.
How common is it?
Because the shoulder joint is the most mobile joint in the body it is potentially unstable. It is therefore the most common joint to dislocate in the body.