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%
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.
- 1. Manaster BJ, May DA, Disler DG. Musculoskeletal imaging. Mosby Inc. (2007) ISBN:0323043615. Read it at Google Books - Find it at Amazon
- 2. Miller TT, Staron RB, Feldman F et-al. The symptomatic accessory tarsal navicular bone: assessment with MR imaging. Radiology. 1995;195 (3): 849-53. Radiology (abstract) - Pubmed citation
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