What is an accessory Navicular Bone?
An Accessory Navicular Bone is an extra bone that may be attached or detached from the navicular bone. It is considered a normal variant and is present in approximately 10% of people. This accessory bone is usually located under the plantar medial aspect of the navicular and is often associated with a pes planus (flatfoot) deformity. Ossification of the navicular bone occurs at three years of age in females and five years of age in males. However, the accessory navicular bone does not begin ossification before eight years of age.
The majority of patients are asymptomatic but, females tend to be more symptomatic. The patient may present with an activity related limp and pain in the arch area. The condition may also be bilateral. During examination, there may be swelling, tenderness, warmth, or redness in the plantar medial aspect of the arch. Relative to a normal foot, a plain x-ray AP view can detect the accessory navicular. An external oblique view is considered to be the best imaging position to detect an accessory navicular bone. An MRI may also be obtained in order to determine the size and type of the accessory navicular as well as assess the posterior tibial tendon.
The accessory navicular is classified into three types. In Type I classifications, the accessory ossicle is mainly in the substance of the posterior tibial tendon and is not attached to the navicular. In Type II, the accessory bone resides very close to the navicular tubercle and is connected to the navicular by a thick layer of cartilage. In Type III classifications, the accessory bone is considered an enlarged navicular tubercle. Type IIIs are essentially a type II that is fused with the navicular by a bony bridge.
In regards to prognosis, when skeletal maturity has been reached, almost all patients become asymptomatic.
Nonoperative treatment usually consists of activity modification, orthotics, or a short leg walking cast. Surgical excision is indicated only after all conservative treatment options have failed.