Shoulder impingement syndrome (rotator cuff injury) occurs when the tendons of the rotator cuff and the subacromial bursa (sac filled with lubricating fluid) are squeezed between the humerus and the acromion. Shoulder impingement syndrome causes persistent pain, especially when reaching up or back. The rotator cuff tendons and bursa can become inflamed causing tendonitis or bursitis. Left untreated, the tendons can thin or tear.