Clinical presentation:
Swelling and discomfort in the left ankle in an adult male from India.

There is a fairly well-defined lytic area in the lateral malleolus and a further area of lysis of the joint margin at its tip. There is irregularity of the bone margins, adjacent to the tibio-talar joint, at the level of the tibio-fibular ligaments. This appearance suggests some external erosion of the adjacent tibia and fibula up to the lowest insertion of the interosseus ligament. The effective working surfaces of the ankle mortice remain intact and this is sometimes a distinguishing feature of this pathology. The soft tissue thickening is visible below the lateral malleolus.