Os tibiale externum

Clinical presentation

35 year old lady with twist injury to the ankle and secondary swelling



Accessory navicular bone measuring 12 x 9 mm present to which the tibialis posterior tendon inserts. Insertional tendinous PD hyperintense signal with minimal fluid in tenosynovial sheath. Diffuse marrow edema in subchondral margins of navicular – accessory navicular articulation. Minimal fluid in ankle joint. Rest of the ankle study is normal.

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Accessory navicular bone (arrows)

Learning points

  • Seen in 10% of the population.
  • Common in adolescents.
  • Common in females.
  • Responsible for medial foot pain

Read more

  1. Classification of accessory navicular bone – Radiopaedia