Acromion is a large bony projection on top of your shoulder blade. Sometimes, the under surface of this bone can have small prominences/or altered shape which impinges on your rotator cuff tendons leading to sub-acromial impingement and thus shoulder pain on over-head movements. This is a procedure in which the under surface of the acromion is re-contoured to increase space between bone and tissue to relieve the impingement symptoms. It is closely related to bursitis of the shoulder, which can occur at the same time. The procedure is done via arthroscopy.