Accessory Navicular

Anatomy

The navicular is one of the tarsal bones in the foot. It is located on the inside, or medial aspect, of the foot, at the arch. The accessory navicular is an ossicle, or extra bone. Some people are born with this ossicle.

The posterior tibialis muscle tendon attaches to the navicular bone. This muscle inverts (turns in) and plantarflexes (points toward the toes) the [Page 7]ankle. This muscle also helps stabilize the medial arch of the foot. The tendon will insert on the accessory navicular, which may cause pain and muscular dysfunction, including loss of the medial arch (flat foot).

The accessory navicular is present at birth, but it is then a soft cartilage. It will begin to ossify (from into a bone) around 9 to 11 years of age. It is usually connected to the navicular with a fibrous union. Some physicians believe that approximately 50% will fuse to the navicular in late adolescence, but this has not been definitively established.

Causes

About 10% of people have an accessory navicular. However, not that many people have symptoms related to it. If the ossicle is large, it can cause a large bump on the medial aspect of the foot. This bump will rub against shoes and can become very painful.

Also, injuries of the fibrous union between the accessory navicular and navicular bone can cause pain. These injuries may destabilize the ossicle, resulting in abnormal motion. This is analogous to a fracture. However, the fibrous tissue connecting the two pieces tends to heal poorly, resulting in continued pain. The attachment of the posterior tibialis muscle causes a constant pull and stress on the ossicle.

If a large portion of the posterior tibialis tendon attaches to the accessory navicular, it can disturb the supportive function of the muscle. This leads to flat foot and an even more prominent accessory navicular bone.

Clinical Evaluation

Many patients will present in childhood with a symptomatic accessory navicular. As the accessory navicular begins to ossify, it may become more symptomatic. An accessory navicular that has no symptoms is not problematic, and treatment should not be initiated unless it becomes symptomatic.