what is acromioclavicular?
<answer> It is the joint formed between the collarbone and the top of the shoulder blade. <context> AC Joint Arthritis. AC Joint Arthritis. The acromioclavicular joint, or AC joint, is the joint formed between the collarbone (clavicle) and the top of the shoulder blade (acromion). Most people have a visible AC joint “bump” on the top of their shoulder where these two bones meet.