Definition: An os trigonum is a small, round bone that sits just behind the ankle joint. The os trigonum is present in about 5-15% of people. An os trigonum occurs when one area of bone does not fuse with the rest of the talus (ankle bone) during growth.
The most common reason people learn they have an os trigonum is that they have an x-ray to help diagnose ankle pain. The truth is that the os trigonum is seldom the cause of the ankle pain.
Normally, having an os trigonum is of no consequence. In some people, however, this small bone can get pinched in the back of the ankle, and even prevent normal motion. This is most commonly seen in ballet dancers who assume pointe and demipointe positions. These positions maximally plantarflex the ankle (point the toes down), and can cause the os trigonum to become pinched in the space behind the ankle.
Also Known As: Posterior Ankle Impingement