Types of Accessory Navicular Bones

Coughlin has described 3 main types of accessory navicular bones:

• Type I ("os tibiale externum"): Up to 30% of cases. A 2–3 mm sesamoid bone in the posterior tibial tendon.
• Type II ("prehallux," shown in radiograph): 50-60% of cases. A triangular or heart-shaped ossicle that measures up to 12 mm. It arises from the secondary ossification center of the navicular bone and is connected to the navicular tuberosity by fibrocartilage or hyaline cartilage.
  
  Interestingly, the name prehallux was chosen to refer to this bone as the sixth toe seen in vertebrates such as the echidna and opossum. It is also thought that this bone may represent a reinforcement for a weakening and pronating foot arch (as a sort of kick-stand for a pes planus).
• Type III ("cornuate navicular"): The accessory navicular bone is connected to the navicular by a bony ridge.

While considered normal variants, type II and III accessory navicular bones can be associated with symptoms. Posterior tibial tendon tears can occur due to increased stress caused decreased leverage of the medial malleolus on the tendon from a more proximal insertion.

References

• Bernaerts A, Vanhoenacker FM, Van de Perre S, De Schepper AM, Parizel PM. Accessory navicular bone: not such a normal variant. JBR-BTR. 2004 Sep-Oct;87(5):250-2.
• Coughlin MJ. Sesamoid and accessory bones of the foot. In: Coughlin MJ (ed) Surgery of the foot and ankle, 8th edition. 2006. Elsevier, Amsterdam, pp 438–494
• Kidner, FC. The prehallux (accessory scaphoid) in its relation to flat foot. J. Bone Joint Surg. 1929 11(4):831–837.
• Miller TT, Staron RB, Feldman F, Parisien M, Glucksman WJ, Gandolfo LH. The symptomatic accessory tarsal navicular bone: assessment with MR imaging. Radiology. 1995 Jun;195(3):849-53.