Os trigonum

Last revised by Andrew Murphy on 21 Jan 2024

The os trigonum (plural: ossa trigona) is one of the ossicles of the foot and can be mistaken for a fracture. 

It sits posterior to the talus on the lateral foot radiograph and represents a failure of fusion of the lateral tubercle of the posterior process. It is estimated to be present in ~7% of adults 1. Bilateral os trigona are seen in 2% of individuals 3. The ossicle usually forms between 7-13 years of age and fuses with the talus in a majority of patients by approximately age 17 4, otherwise persisting as an os trigonum. 

General imaging differential considerations include:

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