A rugby player fell onto his shoulder some weeks ago. He now presents with a painful lump about 4 cm in diameter on the top of his shoulder. What has he injured?: (A) Common extensor origin, (B) The acromioclavicular joint, (C) The head of the humerus, (D) The sternoclavicular joint
Answer is B. The acromioclavicular joint is typically injured by a fall on to the point of the shoulder. The result is inflammation then early aggressive arthritis which produces a painful lump just where the shoulder straps of a rucksack press on to the shoulder. The only possible treatment is to excise the joint, but this may not cure the pain.