An accessory navicular is a small bone that sits in the attachment of the tibialis posterior tendon to the bone.  Some people have them and most people don’t.  Most of the time they don’t cause any problems, so you usually don’t notice them.

Ocassionally they can get injured or experience pain from wearing a tight fitting shoe.

If the pressure is removed and the area of the foot protected with and insole, the synptoms often settle. Sometimes a period of rest in a ‘moonboot’ is required.

Very occasionally surgery is required.  The accessory navicular is removed and the tendon reattached directly to bone.  A period of immobilisation is requried after surgery to allow the tendon to firmly attach to bone