An AC injury is an injury to the acromioclavicular (AC) joint, which is where the collarbone (clavicle) meets the end of the shoulder blade (scapula), known as the acromion. The AC joint helps maintain the position of the shoulder and is important for shoulder control, motion, and strength. The joint is stabilized by a capsule and ligaments, and injury occurs when these structures are damaged. AC joint injuries are commonly seen after sporting events, car accidents, falls from a bicycle, and other sports-related activities (e.g. skiing). The injury typically takes place after a sudden fall onto the shoulder. The type of shoulder separation depends on how much you tear the AC joint or coracoclavicular (CC) ligaments that hold the joint in place. The severity of the injury can range from a mild sprain that heals with rest and physical therapy to a severe injury that requires surgery. Symptoms of an AC joint injury include pain, limited motion in the shoulder, swelling, and bruising. A physical examination to diagnose an AC joint injury includes the AC joint compression test. Treatment for an AC joint injury includes rest, ice, immobilization, activity modification, and physical therapy. In severe cases, surgery may be required.