The term acromioclavicular impingement sign has become increasingly popular with the rise of arthroscopic techniques. The acromion impingement sign is when the humeral tuberosity is rotated under the acromion during shoulder abduction and external rotation, which is under the rostro-capital ligament. The greater tuberosity will then make contact with the rostro-capital ligament. Repeated impingement can cause two pathologies: 1) calcification, tearing, and pain in the impacted rostro-capital ligament; and 2) reactive hyperplasia in the impacted greater tuberosity. On X-ray, you can see a bony growth in the greater tuberosity, which is the acromion impingement sign. The impingement sign is related to the degeneration of the ligaments, and also to the way of movement, repeated impingement, and once the subacromial impingement sign occurs, the amount of movement should be reduced. If the symptoms are not relieved by non-surgical treatment, or if the results are not satisfactory, it is recommended to remove the bone superfluity and to remove the subacromial impingement bursa by arthroscopic surgery, and the symptoms will be relieved soon.