Os tibiale externum is another name for an accessory navicular or os navicularis. This is an extra bone on the medial side of the navicular bone.

Curated Resources:
http://podiapaedia.org/wiki/orthopaedics/accessory-bones/os-tibiale-externum/ (PodiaPaedia)
http://www.podiatryabc.com/o-is-for-os-tibiale-externum/ (Podiatry ABC)

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