When the ball is moving abnormally in the socket ( or vice versa), you may feel the ball "slipping" or "catching". It can come partially or totally out of the socket.
This can be very painful and a sense of " dead arm" can be felt. You might become apprehensive to move your arm in certain directions.
Your shoulder can be subluxed ( partially out ) or dislocated ( totally out of the socket).
If it is dislocated you might not be able to put it back in. With subluxations you can sometimes.
There could be damage to the structures of the shoulder, after trauma. Ligaments, muscles or nerves can be damaged. Sometimes the cartilage in the socket or the ball of your arm might get damaged.
In some patients the arm feels unstable without any trauma. You can have loose joints and the muscles around your shoulder might not be working as they should do. Some muscles have a tendency to work too much whilst other muscles are getting lazy. In other words there is a muscle imbalance.
However sometimes it is not clear cut and the above can overlap.
A very specific examination will be carried out to establish the cause.
Several tests can be done if structural damage is suspected. An in-depth assessment will be carried out to see how your muscles around your shoulder are working.