The navicular bone, located on the medial (inside) foot, is sometimes associated with an accessory bone. This accessory bone is most commonly referred to as an accessory navicular or os tibiale externum, but may also be called navicular secundum, prehallux, bifurcate navicular, accessory tarsal scaphoid, extra scaphoid, or a divided navicular. The accessory bone may be fused (attached) to the navicular, or it may be located within the tendon of the posterior tibial muscle, which attaches to the navicualr at this point.
Accessory navicular is present from birth, and results from a secondary ossification center of the bone. This means that there is an additional area of bony growth in the growing foot, which may eventually attach to the main portion of the navicular by either a bony attachment or with cartilage.
Symptoms of an accessory navicular include pain associated with shoes rubbing against the bony prominence, or pain developing from an eversion-type twisting injury. In the event of a twisting injury, the accessory bone may become detached from the main body of the navicular, essentially acting as a fracture of the bone.
Identification of the os tibiale externum, or the accessory navicular, begins with a thorough hirtory and physical examination. Frequently the accessory bone can be felt through the skin, or even seen on gross examination. X-rays are typically required to differentiate the accessory bone from an enlarged navicular or any other type of fracture of the bone. An MRI is not always necessary, but may be ordered if the diagnosis is not definite.
Once the condition is recognized, treatment may begin with conservative therapy, often involving padding the area and preventing it from rubbing against a shoe. In the event of a fracture through the bone, or a detachment of the bone from the main body of the navicular, it may be immobilized in a hard or soft cast. A number of factors play a role in this clinical decision-making.
Eventually, surgical removal of the accessory bone may be required to alleviate the symptoms. Depending on the particular type of accessory navicular, a number of different surgical techniques may be used. This is at the discretion of the treating foot and ankle surgeon. Typically, the offending bone will be removed in order to relieve the symptoms.
Post-operative care generally consists of protection with a cast, or a soft bandage and a surgical boot. Keeping off of the foot is usually indicated in allowing the foot to heal. Some pain and swelling should be expected. Analgesics may be used to alleviate some post-op pain, and elevation, ice, and rest are typically used to remove some of the swelling associated with the surgery. Details of the surgery may be discussed with your foot and ankle surgeon, including post-operative planning and the risks associated with surgery.
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