How To Conquer The Accessory Navicular Bone
In my experience, the Modified Kidner procedure is one of the most reliable operations for reducing arch pain associated with an accessory navicular bone (a.k.a. os tibial externum). You can also use this procedure to treat a prominence at the inner aspect of the arch, which has been caused by an enlarged navicular bone. The most common patients to visit our office with these problems are between the ages of 8 and 15 and are involved in activities like ice skating, ballet and soccer.
What precipitates the pain? It will usually be caused by rubbing of the skate or other footwear against the prominence. You’ll commonly see blisters or a red irritated area. Other symptoms to look for, especially when you’re treating an older child or adult, include an area of pain along the posterior tibial tendon of the arch and fatigue of the legs. Typically, these patients are not able to participate in sports for a lengthy period of time or you’ll hear them complain of pain and/or soreness after extended activities. Most individuals with a prominent navicular area will have tried accommodating this area with a doughnut pad or adjustments to their skate.
During your clinical exam, you may note erythema to the navicular prominence area and a foot that collapses in stance. While it’s common to see flat feet with these patients, this may not always be the case. You will note a significant difference in the off-weightbearing arch compared to the foot in stance, which is lower. These patients will always have pain to the navicular bone, especially at the major insertion of the posterior tibial tendon just proximal and also inferior to the navicular bone. You may also find they have pain on resisted adduction.
It is important to examine the posterior tibial tendon and measure the extent of pain to this tendon proximal to the navicular bone. You can clinically determine the amount of posterior tibial tendon involvement by assessing the degree of swelling, pain on palpation and strength. To evaluate the patient’s strength, have the patient stand and balance on one foot along with rising up on his or her toes.
Usually, you will only need an X-ray to determine the size or type of the accessory navicular bone or the amount of medial navicular tuberosity hypertrophy. Be cognizant of stress fractures which may be duplicated as a hairline fracture or increased calcification. When treating children, always look for avascular necrosis of the navicular (Kohler’s disease). An X-ray of this condition will reveal a flattening of the navicular along with increased bone density.
Keep in mind there are two different types of accessory navicular bones, which you can distinguish by getting a weightbearing AP X-ray of the foot. Dwight has classified type I as a small, round and discreet accessory bone just proximal to the main navicular bone. Geist described the type II accessory bone, which is closely related to the body of the navicular but separated by an irregular plate of dense fibro-cartilage.
What Causes The Painful Arch?
Perhaps the most common of the extra bones in the foot, the accessory navicular bone is estimated to be present in 7 to 19 percent of the population. Zadek and Gold maintained that the bone persisted as a distinct, separate bone in 2 percent of the population. Also be aware that the accessory bone normally fuses completely or incompletely to the navicular. It is this incomplete fusion which allows for micromotion, which, in turn, may cause degenerative changes that can also contribute to the pain.
Looking at the pathomechanics of the enlarged or accessory navicular bone area, it is commonly believed that the posterior tibial tendon loses its vector of pull to heighten the arch. As the posterior muscle contracts, the tendon is no longer pulling straight up on the navicular but must course around the prominence of bone and first pull medially before pulling upward.
In addition, the enlarged bones may irritate and damage the insertional area of the posterior tibial tendon, making it less functional. Therefore, the presence of the accessory navicular bone does contribute to posterior tibial dysfunction.
Know The Differences
Between The Procedures