Shoulder dislocations occur when the head of the humerus (the long bone of the upper arm) completely leaves the glenoid cavity of the scapula (shoulder blade). Although the structure of the shoulder provides a great range of motion for athletic activities, it is also at a higher risk of dislocations. In atraumatic shoulder instability, the head of the humerus slowly, over time, slips out of its socket, without having experienced an injury or a traumatic incident.