Osteochondral Injury of the Talus (Osteochondritis Dissecans, OcD)
The cartilage lining of the anklebone can be bruised when a sprain of the ankle occurs. As the anklebone (the talus) twists inside its box-like housing, the edge of the talus rubs up and hits the end of the tibia. This causes a bruise and leads to softening of the cartilage. It may lead to a small crack in the cartilage and a cyst forming in the talus bone. This we refer to as an osteochondral injury of the talus. An older term for this, which we do not use any longer, is called Osteochondritis Dissecans.
This is the typical appearance of an osteochondral injury of the talus. It is difficult to see on the X-ray.
These are MRI images of the ankle. The round black hole on the right MRI is typical of this problem.