Painful Bump on Inside of Foot: The Accessory Navicular Bone

Posted on 23 Oct 2014 14:27






Some people, possibly beginning in early adolescents, but perhaps later, can develop a painful bump on the side of their foot. The pain may be worse after athletic activity or just normal walking, and walking itself may become painful. This pain may become constant, but it will tend to improve with continued rest. Depending on the size of the bump, it may rub against shoes, or cause pain if the bump is hit by something. Over time, the arch of the foot may be lost and a flat food will develop. What causes this painful bump on the foot? What can be done about it?

This painful foot condition is caused by an extra bone in the foot called the accessory navicular. Only about 10% of people have this bone (4 to 21%), and not all of them will develop any symptoms. The navicular bone is one of the normal tarsal bones of the foot. It is located on the inside of the foot, at the arch. The image below shows the tarsal bones of the foot. The red colored bone is the navicular bone.


navicular-bone-labelled.png

The navicular bone is colored red.
Image by "BodyParts3D, © The Database Center for Life Science



What is an Accessory Navicular?

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 Problems Does it Cause?

One obvious problem with the accessory navicular is that it may be large and stick out from the inside of the foot. This can cause it to rub against shoes and so become quite painful. The fibrous connection between the accessory navicular and the navicualar, as well, is easy to injure, also leading to pain. This is kind of like a fracture, and such injuries cause the bone to move around too easily, leading to pain with activity. When the connection between the bones is injured in this way, the two bones do not always heal properly, so pain may continue unabated. The image below shows and labelled X-ray, depicting the accessory navicular. Once this happens, the bone will probably stick out even more, causing the problems to become worse.


accessory-navicular-xray.jpg

Xray of accessory navicular, right, compared to normal navicular, left.
Notice the thin space between the navicular and the extra accessory
navicular. This is where the two bones are usually connected with
a fibrous union, a site where injury occurs.

Image via Physio-Pedia.com



Another problem with the ossicle is a muscle that normally connects to the navicular bone, called the tibialis posterior. The tibialis posterior runs down the back of the lower leg and down to the foot, attaching to the navicular and cunieform bones, as well as the second, third, and fourth metatarsal bones. This muscle inverts the foot, or turns it inward, an plantar flexes the ankle, or moves the toes away from the ankle, pointing the foot. The muscle also helps stabilize the medial arch. When the accessory navicular is present, a tendon from the tibialis posterior will attach to it. The muscle then constantly pulls on the accessory navicular, which is already unstable. Depending on how large the tendon is that attaches to the ossicle, it may disrupt the muscles supportive role. All this can lead to pain, dysfunction, and loss of the arch of the foot.

What Are the General Symptoms?

  • A visible bony lump on the inner part of the foot, towards the middle, just above the arch of the foot.
  • Redness, swelling, and sensitivity of the bony prominence
  • Pain or throbbing in the middle of the foot and the arch.
  • Difficulty with foot movement and activity.
  • Possible skin callous or skin irritation caused by footwear rubbing over the lump.

Not everyone who has an accessory navicular will develop these problems. When problems do occur, they may begin in early adolescence. The obvious indication is a painful bump on the inside of the foot, which hurts to touch, and causes problems that gradually become worse, and which are aggravated by activity, walking, etc., leading to all the problems discussed here. Pain may be worse towards the end of the day, and continue into the night.

Among adults, symptomatic accessory navicular is more common in women than in men, with onset typical at 40 years of age or greater. Among symptomatic children, the mean age of onset for maels is 6 years, and for females, 4.5 years. In general, symptoms may occur between 2 and 9 years of age.

Whether symptoms develop depends upon the type of ossicle, which is classified into three types.

Types of Accessory Clavicular

Type 1: A very small ossicle (sometimes called a sesamoid) is located entirely within the tibialis posterior tendon. The ossicle is separate from the navicular bone. This type is usually non-symptomatic and when symptoms do occur they are fairly manageable, responding well to conservative care.

Type 2: This is the "classic" type of accessory navicular which has the aforementioned connection to the navicular bone and also part of the tibialis posterior tendon attached. Here, pulling forces on the accessory navicular can cause trauma, pain, and dysfunction.

Type 3: This type is called a "cornuate" navicular, where the accessory navicular is enlarged and is fused to the navicular through a bony union, and a bony lump is present on the inner edge of the foot (the medial arch). Whether this represents and end stage of the accessory navicular is controversial.

If you or your child experiences these symptoms, see your physician, who will probably order X-rays of the foot. In the young, when the ossicle has not calcified enough, it may not show up on an X-ray.

Treatment

Although operative treatment, and removal of the accessory navicular is possible, this is not usually indicated at first. Conservative nonoperative treatment is best, the course depending on the severity of the symptoms. When the pain is very severe, which could indicate a fracture, a period of immobilization might be required. This is done by wearing a fracture boot, or a cast, which can help the ossicle stay stable, aiding in healing. Immobilization usually lasts between 4 to 6 weeks. Afterwards, physical therapy exercise, or any appropriate home course, should be used to help strengthen the ankle and return the ankle and foot to full range of motion, and have no pain on movement. Sometimes crutches are used when weight bearing is too painful, but it is best to try to bear weight when possible.

For less severe symptoms, decreasing or modifying activity, such as avoiding aggravating activities, may suffice. Ice and NSAIDS can be used to help control pain. An arch support or an orthotics may help to stabilize the arch during this time. When rubbing on the bump causes pain, a doughnut pad can be worn. Exercises to increase range of motion and improve movement should still be used.

When severe symptoms do not improve with conservative care, and operation may be indicated. Several operations are used, with or without reattachment of the tibialis posterior tendon. Discuss your choices with your orthopedic surgeon.

References
1. Micheli, Lyle J. Encyclopedia of Sports Medicine. Thousand Oaks, CA: SAGE Publications, 2011.
2. Bracker, Mark D. The 5-minute Sports Medicine Consult. Philadelphia: Lippincott Williams & Wilkins, 2001.


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This page created 23 Oct 2014 14:27
Last updated 03 Jun 2016 22:34

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