What is the Accessory Navicular?
The accessory navicular (os navicularum or os tibiale externum) is an extra bone or piece of cartilage located on the inner 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.
What is Accessory Navicular Syndrome?
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 tibial tendon are aggravated. This can result from any of the following:
- Trauma, as in a foot or ankle sprain
- Chronic irritation from shoes or other footwear rubbing against the extra bone
- Excessive activity or overuse
Many people with accessory navicular syndrome also have flat feet (fallen arches). Having a flat foot puts more strain on the posterior tibial tendon, which can produce inflammation or irritation of the accessory navicular.