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 normal feet. An os trigonum occurs when one area of bone does not fuse with the rest of the talus (ankle bone) during growth.
Normally, having an os trigonum is of no consequence. In some people, however, this small bone can get caught in 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.
Other causes of posterior ankle impingement include tendonitis, fracture, and ligament injuries.
Also Known As: Posterior Ankle Impingement
