Os Tibiale Externum

Os tibiale externum (accessory navicular) is a large ossicle adjacent to the medial side of the navicular bone. The tibialis posterior tendon often inserts with a broad attachment onto the ossicle, which may cause a painful tendinosis due traction between the ossicle and the navicular. Such changes are best seen on MRI.
Reference: Manaster BJ et al. Musculoskeletal Imaging: The Requisites, 3rd edition. Mosby Elsevier 2007
Credit: Dr Donna D'Souza
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