Acromion is a lateral process out of the shoulder blade of the shoulder joint.  On x-rays, it has been classified into four types.  Type II is the most common type and is gradual curve along the head of the shoulder.  Type II acromion may occasionally cause impingement of the shoulder joint.  The type III acromion is the one which is usually involved with impingement syndrome of the shoulder joint as it is curved like a hook over the rotator cuff and lead to injury and possible tearing of the rotator cuff.