Can you tell me what exostosis means? X-rays taken on our daughter show there is exostosis causing loss of ankle motion. We're not sure what that means.

Exostosis (plural: exostoses) is the formation of new bone on the surface of a bone. It is an overgrowth of bone. These nodules or bumps on various bones of the body can cause pain and other problems when they press on nearby tissues (especially nerves or blood vessels).

It is a benign condition, meaning that it is not bone cancer. But even one exostosis can cause problems. Symptoms depend on where the nodules are located and what size and shape they are. They can limit movement of joints, causing chronic pain that can be mild to severe and even disabling. In some cases, they must be surgically removed.

There is a hereditary condition seen in children called multiple cartilaginous exostoses (MCE). In MCE, there are many bony projections or exostoses. They are covered or capped by cartilage. The bones affected most often are the long tubular bones of the forearm and lower leg. Sometimes flat bones, vertebrae, ribs, and short tubular bones can be affected, too.

Deformity of the bones in the lower leg is often accompanied by an ankle problem called ankle valgus. Valgus refers to the way the side of the ankle that is closest to the other leg drops down toward the floor. This happens because the tibia (larger of the two lower leg bones) is tapered at the bottom. At the same time, the fibula (smaller bone on the outside of the lower leg) is shortened. When the bones shift in response to these changes, ankle valgus occurs. A second potential problem in MCE is deformity of the forearm bones.

Your daughter may simply have one exostosis that is causing a problem and not the more involved condition of MCE. But if ankle motion is limited, it may be necessary to remove the bony overgrowth. Th would require surgery. If you have had X-rays done, then you are probably under the care of a physician. He or she will be able to assess the full extent of the problem and advise you accordingly.