Separated shoulder, also known as acromioclavicular (AC) joint separation, is a sprain of the ligaments that attach the clavicle (collarbone) to the acromion (highest part of the scapula, or shoulder blade). The sprain can be either partial, with minimal separation of the clavicle and acromion, or complete, which means the bones have come A shoulder separation is not truly an injury to the shoulder joint. The injury actually involves the acromioclavicular (AC) joint. The AC joint is where the collarbone (clavicle) meets the highest point of the shoulder blade (acromion). A separated shoulder involves injury to the ligaments that hold your collarbone to your shoulder blade. In a mild separated shoulder, the ligaments might just be stretched. In severe injuries, ligaments may be torn. The most common cause of a separated shoulder is a blow to the point of the shoulder or a fall directly on the shoulder. ZxHb.