The sterno-clavicular joint forms between the collarbone (clavicle) and breastbone (sternum).  Injury to the ligaments stabilizing the joint causes instability and the collarbone sticks out on every movement of the shoulder. This injury can be sports related or due to accident or fall. This may be associated with pain and obvious deformity, it may also compromise sporting activities. Your surgeon may advise surgery to stabilize to the sternoclavicular joint.