Accessory Navicular

Accessory navicular is an accessory ossicle of the foot which is located on the medial side of foot, proximal to the navicular and in continuity with the tibialis posterior tendon.  Approximately 2-12% of people may have this condition.  Accessory navicular generally does not ossify until 9 years of age, and in about one half of cases, the accessory navicular will go on to fuse to navicular

A valgus stress injury may fracture the attachment of the ossicle to the navicular resulting in abnormal motion.  The insertion of a major portion of the tibialis posterior tendon into the accessory bone displaces the tendon, allowing the foot to deviate into a valgus position.  This results in a flatfoot with prominences of the accessory bone and the navicular.

Also see Pes Planus and Posterior Tibialis Tendinitis