The acromion-clavicular (AC) joint is where the shoulder blade (acromion) and the collar bone (clavicle) attach. They are connected via ligaments. When the ligaments and of the AC joint are injured, this is referred to as an AC joint sprain. During certain activities stretching forces are placed on the AC joint, if these forces are beyond what the AC joint can withstand, tearing of the ligaments may occur. An AC joint sprain can range in severity from a small sprain with minimal pain, to a severe sprain with high pain levels loss in function.