The AC (acromioclavicular) joint is the joint between the collarbone (clavicule) and the roof of the shoulder (acromion).
The AC joint can become inflamed; it then becomes painful. The inflammation can sometimes be associated with one form of rheumatism or another. Sometimes, there is an underlying injury that sustains the inflammation: cartilage injuries, meniscus injuries, avascular necrosis of the end of the collarbone, osteolysis, etc.
Read more about AC injuries here
