Shoulder instability refers to when the structures acting to centralise the ball in the socket joint are not effectively maintaining this. This can occur when the upper arm bone is forced out of its socket, partially (known as a subluxation) or fully (known as a dislocation). This can cause ligaments, tendons and muscles around the joint to tear and loosen and therefore dislocations can occur repetitively. Shoulder instability can also be caused by congenital joint features or repetitive actions in certain positions.