The main shoulder joint is ball and socket type joint. The shoulder blade has a socket type structure at the end and into this fits the ball shape at the top of the arm bone (humerus). The two bones are held together by ligaments and the muscles connect to the bones via the tendons.

The muscles are large and the ligaments and tendons are strong to allow the huge range of movement the arm enjoys. All the while the ball is held tightly into the socket.

Where the ball becomes loose and feels to the patient like it might fall out at any moment, the patient is suffering from shoulder instability.

There are three main groups of people who might suffer this condition.

1. Some people are born with abnormalities in their ligament and tendon arrangement. In everyday language, we call these people “double-jointed”. They have a looser arrangement than the rest of us and whilst this might at times be an advantage, if it appears in the shoulder, it could lead to shoulder instability.
2. People who have fully dislocated a shoulder are far more likely to suffer shoulder instability. Once ripped apart by a dislocation (when the ball comes clean out of the socket) the ligaments and tendons may never be the same again. The younger the patient is when this trauma first happens, the more likely they are to suffer later complications.
3. Some people born with perfectly adequate shoulder ligaments and tendons find that they weaken them. This is common in some sports such as tennis or baseball pitching where the arm is repeatedly brought over the shoulder in a way most of us do rarely.

This is one condition which becomes less common amongst older people who are likely to have slightly stiffer joints anyway. If you feel you need further advice on your shoulder injury you should consult an Orthopedic doctor immediately.