The accessory navicular is a congenital anomaly, meaning that you are born with the extra bone. As the skeleton completely matures, the navicular and the accessory navicular never completely grow, or fuse, into one solid bone. The two bones are joined by fibrous tissue or cartilage. Girls seem to be more likely to have an accessory navicular than boys.
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.
Many people have accessory (?extra?) naviculars (figure 1) - a prominent extra bone extending from the navicular bone. Most accessory naviculars are completely asymptomatic. However, some individuals will develop pain on the inside of their midfoot. Pain may occur from the pressure of the shoe ware against the prominence, irritating either the bone itself or the fibrous junction where the accessory bone meets the regular navicular. Alternatively, the fibrous junction or interface may become painful as a result of tension applied by the posterior tibial tendon through its connection or insertion at that site. Often, individuals will be asymptomatic for years, however, a new pair of shoes or a change in their activity level can cause symptoms. The accessory navicular itself typically develops during adolescence, when the two areas of the navicular bone fail to fuse together.
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 How do I stretch my Achilles tendon? 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.
Non Surgical Treatment
The initial treatment approach for accessory navicular is non-operative. An orthotic may be recommended or the patient may undergo a brief period of casting to rest the foot. For chronic pain, however, the orthopedic surgeon removes the extra bone, a relatively simple surgery with a brief rehabilitation period and a very good success rate.
Depending upon the severity the non operative or conservative treatment should be maintained for at least 4- 6 months before any surgical intervention. There are 2 surgeries that can be performed depending upon the condition and symptoms. First is simple surgical excision. In this generally the accessory navicular along with its prominence is removed. In this procedure, skin incision is made dorsally to the prominence of accessory navicular. Bone is removed to the point where the medial foot has no bony prominence over the navicular, between the head of the talus and first cuneiform. Symptoms are relieved in 90% of cases. Second is Kindler procedure. In this the ossicle and navicular prominence is excised as in simple excision but along with the posterior tibial tendon advancement. Posterior tibial tendon is split and advanced along the medial side of foot to provide support to longitudinal arch. After surgery 4 week short leg cast, well moulded into the arch with the foot plantigrade is applied. Partial weight bearing till the 8th week and later full weight bearing is allowed. When the cast is being removed can start building up the ROM to counter atrophy and other physical therapy treatment which include stretching and strengthening exercises.
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