The shoulder is a joint which has a huge range of motion but this means that it is inherently unstable and relies on the surrounding structures (muscles and ligaments) to stop it dislocating. If any of these structures is damaged the shoulder can become unstable. The shoulder can be unstable in any direction. I will only consider the anterior variations (the most common) here.
There are several different types of instability
- Muscular Imbalance
It is possible to move from one type of instability to another with time and there is a spectrum in-between these points.