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Os tibiale externum

Dr MT Niknejad and Dr Frank Gaillard et al.

An os tibiale externum (or accessory navicular / os naviculare) 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. Most cases are asymptomatic but in a small proportion it may cause a painful tendinosis due traction between the ossicle and the navicular. Such changes are best seen on MRI.

an accessory navicular bone is located posterior to the posteromedial tuberosity of the tarsal navicular bone.
an accessory navicular bone is present in 4 - 21% of the population.
first appears in adolescence.
more common in female patients.
reported prevalence of bilaterality varies from 50 - 90%

Classification

Th Geist classification divides these into 3 types.

type 1 accessory navicular bone
- also known as os tibiale externum
- 2 - 3 mm sesamoid bone embedded within the distal portion of the posterior tibial tendon.
- It does not have a cartilaginous connection to the naviculam tuberosity and may be separated from it by up to 5 mm.
- accounts for 30% of accessory navicular bones
- usually asymptomatic
type 2 accessory navicular bone
- triangular or heart-shaped
- measures up to 12 mm
- accounts for 50 - 60% of all ossicles.
- connected to the naviculam tuberosity by a 1-2-mm thick layer of either fibrocartilage on hyaline cartilage.
- eventual osseous fusion to the navicular tuberosity may take place
type 3 accessory navicular bone
- an especially prominent navicular tuberosity called a cornuate navicular
- thought to represent a fused type 2 and is occasionally symptomatic as a result of painful bunion formation over the bony protuberance.