The os trigonum is a tiny bone found on the posterolateral aspect of the talus. It has a separate ossification center which does not unite with the talus itself. It is difficult to make the diagnosis, as clinical features and physical examination findings mimic those that occur with conditions related to flexor hallucis longus, an accessory soleus muscle, peroneal tendons, tibialis posterior, or arthritis of the posterior subtalar or tibiotalar joints. Misdiagnosis may lead to inadequate management which includes early weight bearing and unrelieved symptoms. This syndrome is found mostly in ballet dancers, runners, and soccer players.
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