What is it?
An accessory navicular can be a source of medial mid foot pain.  There are three types of accessory navicular bones.  Type I accessory navicular is a sesamoid bone which is not usually symptomatic. Type II is a larger structure that is triangular or heart shaped.  It is a synchondrosis and is the most common.  Type III accessory navicular is considered the end stage of type II when fusion of a larger secondary ossification center results in a cornuate navicular.  Patients typically complain of pain on the medial side of the foot and pain wearing shoes.  There is usually palpable, localized tenderness at the site of the navicular.  It is not uncommon to have bilateral accessory naviculars.  It is more common in young, active females.  Radiographs (external oblique view) and even better, bone scans can be helpful in diagnosis.  Differential diagnosis: tarsal tunnel syndrome/tarsal coalition, injury to deltoid or spring ligament, posterior tibialis tendon dysfunction, avulsion of posterior tibialis, stress fracture to navicular.
Conservative treatment includes immobilization, anti-inflammatories, and orthotics.  If pain persists, then surgical excision is recommended.  Post-surgical treatment regularly includes a time of immobilization and non-weight bearing.  After that time, physical therapy may be prescribed.  Intervention may include: support to the foot, joint and tissue mobilization, balance and proprioception retraining, and strengthening.
References: Grogan DP, Gasser SI, and Ogden JA.  The painful accessory navicular: A clinical and histopathological study.  Foot & Ankle, 1989; 10(3): 164-169.
Romanowski ACJ and Barrington NA.  The accessory navicular – An important cause of medial foot pain.  Clinical Radiology, 1992; 46: 261-264.
Requejo SM, Kulig K, and Thordarson DB.  Management of foot pain associated with accessory bones of the foot: Two clinical case reports.  Journal of Orthopaedic & Sports Physical Therapy, 2000; 30(10): 580-594.