The accessory navicular also termed the os navicularum or os tibiale externum - is an extra bone or piece of cartilage on the inner side of the foot above the arch that attaches to the posterior tibial tendon within this area. This extra bone, present at birth, is not part of the normal bone structure and found in approximately 10% of the population. Some people with an accessory navicular may be unaware of the condition if symptoms are never experienced. But accessory navicular syndrome is a painful condition caused by aggravating the bone, the posterior tibial tendon or both.Accessory navicular syndrome is an irritation of the accessory navicular and/or posterior tibial tendon. This irritation can be caused by shoe rubbing, trauma, excessive activity, or overuse and can cause problems with the shape and function of your foot. Many people with this disorder also have flat feet which puts more strain on the posterior tibial tendon. Some people are born with an accessory Navicular because during development, the bones of the feet sometimes develop abnormally causing the extra bone to form on the inside of the foot.
People who have an accessory navicular often are unaware of the condition if it causes no problems. However, some people with this extra bone develop a painful condition known as accessory navicular syndrome when the bone and/or posterior tibial tendon are aggravated. This can result from any of the following. Trauma, as in a foot or ankle sprain. Chronic irritation from shoes or other footwear rubbing against the extra bone. Excessive activity or overuse.
Adolescence is a common time for the symptoms to first appear. This is a time when bones are maturing and cartilage is developing into bone. Sometimes, however, the symptoms do not occur until adulthood. The signs and symptoms of accessory navicular syndrome include a visible bony prominence on the midfoot (the inner side of the foot, just above the arch) Redness and swelling of the bony prominence. Vague pain or throbbing in the midfoot and arch, usually occurring during or after periods of activity.
To diagnose accessory navicular syndrome, the foot and ankle surgeon will ask about symptoms and examine the foot, looking for skin irritation or swelling. The doctor may press on the bony prominence to assess the area for discomfort. Foot structure, muscle strength, joint motion, and the way the patient walks may also be evaluated. X-rays are usually ordered to confirm the diagnosis. If there is ongoing pain or inflammation, an MRI or other advanced imaging tests may be used to further evaluate the condition.
Non Surgical Treatment
Excess weight will increase the force on the posterior tibial tendon as it inserts into the accessory navicular and will tend to precipitate or aggravate symptoms. If a patient with a symptomatic accessory navicular is overweight, then losing weight can be very helpful. Even losing 5-10lbs will decrease the amount of force going through the foot with each step by as much as 15-30lbs. This is because the foot acts like a lever serving to magnify the force absorbed by the foot with each step.
Surgery may be an option if non-surgical treatment does not decrease the symptoms of accessory navicular syndrome. Since this bone is not needed for the foot to function normally, Your surgeon may remove the accessory navicular, reshape the area, and repair the posterior tibial tendon for improved function.