Melorheostosis is a rare bone disease. It causes the abnormal growth of new bone tissue on the surface of existing bones. The new bone has a characteristic appearance on x-rays, often described as “flowing” or like dripping candle wax. The excess bone growth typically occurs on the bones in one arm or leg, although it can also affect the pelvis, breastbone (sternum), ribs, or other bones. (The term “melorheostosis” is derived from the Greek words “melos,” which means limb; “rheos,” which means flow; and “ostosis,” which refers to bone formation.) The abnormal bone growth associated with melorheostosis is noncancerous (benign), and it does not spread from one bone to another.
The signs and symptoms of melorheostosis usually appear in childhood or adolescence. The condition can cause long-lasting (chronic) pain, permanent joint deformities (contractures), and a limited range of motion of the affected body part. In some people, the limb may appear thickened or enlarged, and the skin overlying the affected area can become red, thick, and shiny.
Another rare disease, Buschke-Ollendorff syndrome, can include melorheostosis. Buschke-Ollendorff syndrome is characterized by skin growths called connective tissue nevi and areas of increased bone density called osteopoikilosis. A small percentage of affected individuals also have melorheostosis or other bone abnormalities. Scientists originally speculated that melorheostosis that occurs without the other features of Buschke-Ollendorff syndrome might have the same genetic cause as that syndrome. However, it has since been determined that Buschke-Ollendorff syndrome and melorheostosis that occurs alone are caused by mutations in different genes, and the two conditions are considered separate disorders.