The os trigonum is a small accessory ossicle posterior to the lateral talar posterior process, present in approximately 7 to 10 percent of the population as a separate bone when the lateral talar posterior tubercle fails to fuse during adolescent development. It is usually asymptomatic but can become a source of posterior ankle pain in ballet dancers and soccer players who work in forced plantarflexion, where the os trigonum is compressed between the posterior tibia and calcaneus (os trigonum syndrome).
Os trigonum syndrome produces posterior ankle pain specifically reproduced by forced passive plantarflexion (the plantarflexion impingement test) and by rising onto pointe in ballet. Ultrasound or MRI confirms the ossicle and associated soft tissue impingement. Conservative management with restriction from plantarflexion activities and corticosteroid injection resolves most cases; surgical excision of the os trigonum is performed for refractory impingement.
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