Os trigonum

Last revised by Dr Joshua Yap on 13 Jul 2022

The os trigonum (plural: os 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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Cases and figures

  • Figure 1
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  • Case 1: os trigonum
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  • Case 2: os trigonum
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  • Case 3: os trigonum
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  • Case 4: os trigonum (T1W MRI)
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  • Case 5
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  • Case 6
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  • Case 6: with bilateral subtalar osteoarthritis
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  • Case 7: os trigonum syndrome
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  • Case 8
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  • Case 9
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