Not such a normal variant JBR-BTR 2004 87(5)

In Accessory Ossicles and Sesamoid Bones: Spectrum of Pathology and Imaging Evaluation Dr. Kalantari; Dr. Seeger; Dr. Chow; Dr. Motamedi Appl Radiol. 2007;36(10):28-37.

Core Curriculum
Objectives
1. Describe the clinical symptoms associated with accessory
2. Discuss imaging studies useful for the evaluation of accessory navicular
3. Discuss treatment of accessory navicular

The accessory navicular is an anatomic variant that occurs in approximately 10% of individuals. It is characterized by an accessory ossification center adjacent to the navicular at the site of the tibialis posterior tendon attachment. It is frequently associated with pes planus and can occasionally be symptomatic.
There are 3 types of accessory navicular. Type I is a small accessory ossicle in the tendon. Type II is larger and articulates with the navicular. This synchondrosis or synovial articulation can degenerate and cause symptoms. Type III is a fused accessory navicular.

accessory navicular is an accessory ossicle of the foot which is located on the medial side of foot, proximal to the navicular and in continuity with the tibialis posterior tendon;
- approximately 2-12% of people may have this ossicle;
- accessory navicular generally does not ossify until 9 years of age, and in about one half of cases, the accessory navicular will go on to fuse to navicular;
- a valgus stress injury may fracture the attachment of the ossicle to the navicular resulting in abnormal motion;
- insertion of major portion of tibialis posterior tendon into accessory bone displaces tendon, allowing foot to deviate into a valgus position;
- this results in flatfoot w/ prominences of accessory bone& navicular;

Matthew B Dobbs, MD and Tim Walton, RN, BSN Iowa Orthop J. 2004; 24: 84–85.
The accessory navicular bone is one of the most symptomatic bones of the foot. Although it has been reported to be present in various members of the same family, there is a lack of knowledge about its inheritance pattern. We report two large pedigrees in which accessory navicular is inherited in an autosomal dominant fashion with incomplete penetrance.

Surgical Pearls: How To Conquer The Accessory Navicular Bone - By Richard T. Braver, DPM
the Modified Kidner procedure is one of the most reliable operations for reducing arch pain associated with an accessory navicular bone (a.k.a. os tibial externum). You can also use this procedure to treat a prominence at the inner aspect of the arch, which has been caused by an enlarged navicular bone.

Korean J Radiology 2004: 5; 274 Choi et al
The MR imaging findings are a persistent edema pattern in the accessory navicular bone and within the synchondrosis indicating osteonecrosis, inflammation and destruction of the cartilage cap. Posterior tibial tendon dysfunction was clinically evident in most patients.

This chapter on normal variations of the pediatric foot discusses accessory navicular. Os trigonum will be discussed at a later date.

The accessory navicular is occasionally the source of pain and local tenderness over the instep. If conservative measures fail, surgical treatment may be required and the results of 62 operations to one or both feet in 47 patients are reported. Twenty-six patients were treated by the Kidner operation, in which the main insertion of the tibialis posterior is re-routed; in the remaining 21 the ossicle was merely excised. Excision was as effective as the Kidner technique, provided that the medial surface of the main navicular bone was contoured to prevent any residual prominence. Both procedures were successful in relieving symptoms in the majority of cases and failures resulted from errors in the selection of patients or in the surgical technique. Correction of any associated flat foot was secondary to growth and maturation of the foot rather than to the operation; hence the Kidner procedure does not confer any particular advantages over simple excision.
MF Macnichol and S Voutsinas JBJS B 1984 Vol 66-B, Issue 2, 218-226