An Acromioclavicular Joint Separation or AC Joint Separation or AC Joint Sprain or simply Shoulder Separation is a common shoulder injury among active people like athletes. In this injury, the clavicle (collar bone) separates from the scapula (shoulder blade). The clavicle and scapula together form the socket that holds the ball of the upper arm bone. AC Joint Separation is generally caused due to a direct injury like a fall or a blow to that spot. It is a common sports injury, frequently seen in football and hockey players and cyclists.