The accessory navicular is an extra bone that is found on the medial side of the foot in some people. There are several different types of these extra pieces of bone.
The treatment of the problems that are associated with an accessory navicular will depend of the nature of the symptoms.
If the conservative measure fail, then surgery is an option. These options include a surgical removal of the extra bone and a remodeling of the insertion of the posterior tibial tendon. This may or may not also include a surgical reconstruction of any associated flat or pronated foot.
See also:
http://www.footdoctortoday.com/the-accessory-navicular/34/
http://www.podiatryexperts.net/uapage/11