Acromioclavicular joint injuries are usually the result of a direct hit to the shoulder which can rupture ligaments such as the coracoclavicular and acromioclavicular ligaments.

In the present case, the positive piano key sign upon physical examination raised suspicion of an acromioclavicular dislocation. The coracoclavicular and acromioclavicular distances are important measurements in the evaluation of their respective ligaments which may be injured when an acromioclavicular joint injury is suspected.

Normal range of the CC distance is 11-13 mm.

Normal range of the AC distance is 1-3 mm.

Widened distances indicate an acromioclavicular joint injury.