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?

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.