This document discusses painfull arc syndrome, also known as impingement syndrome of the shoulder. It describes the history and evolution of understanding of this condition. Impingement syndrome occurs when the rotator cuff tendons are pinched between the head of the humerus and the acromion process during arm elevation. There are three main types of impingement: primary, secondary, and subcoracoid. Treatment involves conservative measures like medication and physical therapy initially but may require arthroscopic or open surgery like acromioplasty in resistant cases. Complications of acromioplasty can include infection, nerve injury, or loss of shoulder function.