Some people have more bones in their feet than others. Actually, it?s not all that uncommon to have extra bones in the feet. These extra bones area called accessory bones. The navicular bone, one of the small bones located at the instep or arch of the middle of the foot, is an example of an extra bone people are born with. It?s called the accessory navicular bone. During the maturation process, the navicular and the accessory navicular never fuse into one solid bone, but remain connected by fibrous tissue or cartilage. It is estimated that 4-14% of the population are born with an accessory navicular bone.
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.
Plain x-rays are used to determine the size of the accessory navicular. There are three main types of accessory navicular bones: a small bone embedded within the nearby posterior tibial tendon; a triangular shaped bone connected to the navicular by thick cartilage; and a large prominent navicular tuberosity thought to represent an accessory navicular that has fused to the navicular. If the status of the posterior tibial tendon needs to be assessed or if other problems are suspected, (ex. Navicular stress fracture) it may be necessary to perform an MRI. Although this Why is my Achilles tendon burning? not considered routine, an MRI may be helpful in identifying the degree of irritation. An MRI would demonstrate fluid or edema that may accumulate in the bone as a result of the irritation.
Non Surgical Treatment
Most cases of accessory navicular syndrome may be treated conservatively with some sort of immobilization. This should allow the fibrous tissue between the two bones to heal. If a patient is extremely flat footed (pronated) then I lean more towards an orthotic than a boot as my main goal is to keep the patient's foot from flattening out too much and thus reduce the strain on the two bones. Supplementation with ice, oral anti-inflammatory medication. If the patient is athletic sometimes we can keep them active with an orthotic, but other times they have to give up their sport for a period of time to allow the condition to heal.
For patients who have failed conservative care or who have had recurrent symptoms, surgery can be considered. Surgical intervention requires an excision of the accessory navicular and reattachment of the posterior tibial tendon to the navicular. Often times, this is the only procedure necessary. However, if there are other deformities such as a flat foot or forefoot that is abducted, other procedures may be required.
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