The accessory navicular (os navicularum or os tibiale externum) is an extra bone or piece of cartilage located on the innter side of the foot just above the arch. It is incorporated within the posterior tibial tendon, which attaches in this area. An accessory navicular is congenital (present at birth). It is not part of normal bone structure and therefore is not present in most people.

People who have an accessory navicular often are unaware of the condition if it causes no problems. However, some people with this extra bone develop a painful condition known as accessory navicular syndrome when the bone and/or posterior tibal tendon are aggravated. This can result from any of the following: Trauma, Chronic irritation from shoes or other footwear, excessive activity or overuse