Dr. Ebraheim’s educational animated video describes the accessory navicular bone.

What is an accessory navicular bone?
It is an extra bone that may be attached or detached from the navicular bone. It is considered a normal variant present in 10% of people. It is usually located under the plantar medial aspect of the navicular. It is often associated with a pes planus (flatfoot deformity). Ossification of the navicular bone occurs at three years of age in females and at 5 years of age in males. The accessory navicular bone does not begin ossification before eight years of age.
Presentation
The majority of patients are asymptomatic. Females are usually more symptomatic. The patient may present with an activity related limp and pain in the arch area. The condition may also be bilateral.
Examination
There could be swelling, tenderness, warmth or redness in the plantar aspect of the arch.
Radiology
An external oblique view is considered the best view to detect an accessory navicular bone. MRI may be helpful. An MRI can also be obtained to determine the size and type of the accessory navicular as well as assess the posterior tibial tendon.
Classification
•Type I: the accessory ossicle is mainly in the substance of the posterior tendon and is not attached to the navicular.
•Type II: the accessory bone resides very close to the navicular tubercle and is connected to the navicular by a thick layer of cartilage.
•Type III: the accessory bone is considered an enlarged navicular tubercle. It is essentially type II that is fused with the navicular by a bony bridge.
Prognosis
When skeletal maturity is reached, almost all patients become asymptomatic.
Treatment
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. The procedure is simply excision of the accessory navicular.

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Background music provided as a free download from YouTube Audio Library.
Song Title: Every Step