Clavicular hook plate is known to be an effective treatment on acromioclavicular (AC) joint injury, but there have been some reports of complications, like osteolysis and bony erosion of the undersurface of acromion. Fifty-five year old male underwent open reduction and hook plate insertion on Rockwood type 5 acromioclavicular joint dislocation. He complained of protrusion of posterior acromion at 1 month after the surgery, and acromial fracture was noted in simple radiographs. The hook plate was removed and any other treatment for osteosynthesis was refused by the patient. At the 18 months after the surgery, the patient had no pain and a full range of motion with no tenderness around the shoulder joint. After two years, plain radiographs revealed complete bony union of the acromion fracture.