The accessory navicular is an ossicle, or extra bone located medially to the navicular. Depending on the type, or stage, it may be connected to the navicular by a fibrous union, via a type of joint called a synchrondrosis. In those who have this extra bone, it is present at birth, but it starts as soft cartilage and then begins to ossify (turn into bone) at around age nine. Some sources believe that, in about half of those who have it, the bone will fuse to the navicular in late adolescence, but it is not clear that this actually happens.
What causes pain in the Achilles tendon?
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.
The primary reason an accessory navicular becomes a problem is pain. There is no need to do anything with an accessory navicular that is not causing pain. The pain is usually at the instep area and can be pinpointed over the small bump in the instep. Walking can be painful when the problem is aggravated. As stated earlier, the condition is more common in girls. The problem commonly becomes symptomatic in the teenage years.
Upon examining a foot with a symptomatic accessory navicular, there will often be a bony prominence on the inside of the foot, just below and in front of the inside ankle bone (medial malleolus). The size of this prominence will vary from small to quite large. Pressing over this area will often cause discomfort. There may be an associated flat foot deformity as this can occur in patients with an accessory navicular. Stressing the posterior tibial tendon by raising the heel up and down on one foot, or by forcing the foot to the inside against resistance, may aggravate the symptoms as these maneuvers stress the posterior tibial tendon which is attached on the inside of the accessory navicular bone. Patients may walk with a slight limp due to the pain.
Non Surgical Treatment
Treatment of the accessory navicular begins with rest, which may include activity modification or temporary immobilization in a boot or a brace. Once the inflammation subsides the foot needs to be supported. The support consists of a specially designed orthotic arch support. Occasionally, the orthotic will dig into the edge of the accessory navicular bone under the arch of the foot. This is very uncomfortable. For this reason the orthotic support needs to be carefully made. The orthotic support will help control (but not cure) the flat foot and will often decrease the inflammation on the navicular.
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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