 When the arm is raised, the subacromial space (gap between the anterior edge of the acromion and the head of the humerus) narrows; the supraspinatus muscle tendon passes through this space.Anything that causes further narrowing has the tendency to impinge the tendon and cause an inflammatory response, resulting in impingement syndrome.Such causes can be bony structures such as subacromial spurs (bony projections from the acromion), osteoarthritic spurs on the acromioclavicular joint, and variations in the shape of the acromion.