Ankle Deformity in Children with Bone Overgrowth

Little by little, doctors are learning more about an ankle condition called multiple cartilaginous exostoses (MCE). The results of this study added new information about MCE.

First, what is multiple cartilaginous exostoses (MCE)? And what causes it? Overgrowth of bone is called exostosis or exostoses (plural form). 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.

The condition is inherited and is present in childhood. 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.

Using the knowledge available so far, researchers have tried to come up with a way to classify MCE. It's called the Taniguchi classification. This classification method is based on the presence of three groups. Group I is MCE alone. Children in group II have MCE with forearm involvement but the bones in the forearm (radius and ulna) are normal in length. And Group III refers to children with MCE and shortening of the radius and ulna. Alternately stated, these groups could be labeled mild, moderate, or severe. The more severe cases have worse valgus deformity with many more bone growths (exostoses).

New research shows that there are two gene mutations associated with MCE (EXT1, EXT2). Children with EXT2 mutations have more severe cases of MCE. But not all children have these mutations, so they couldn't classify MCE using gene mutations. The authors of this study go the next step to look for links between the ankle and forearm deformities to help understand this problem.

They were able to find 33 children with MCE to study. None of the children had any surgery to correct the problem. X-rays were used as the main measuring tool.

The children were all assigned to a classification group (I, II, III). Angle measurements of the ankle were calculated using the X-rays. The location of the exostoses was also categorized by the bone affected (tibia or fibula) and the location of the bone growths on each bone (medial or lateral). Medial always refers to the side closest to the body midline (or in the case of ankle MCE, the other leg). Lateral is the side away from the other leg.

The authors looked for other factors that might predict the presence of both bone deformities (ankle valgus and forearm deformity/shortening). For example, age and gender were considered as possible contributing factors. They also compared degree of ankle valgus deformity within each of the three Taniguchi groups.

They found a distinct correlation (link) between Taniguchi Group III and ankle valgus. Most of the extra bone growth was between the two bones of the lower leg (tibia and fibula). Boys were affected more often and with greater severity compared with girls.

Shortening of the fibula was present in all Group III children. The result was severe ankle valgus. At the same time, the researchers noticed that children in Group III with the shorter fibulae also had shortening of the ulnar bones in the forearm. The fibula in the lower leg and ulna in the forearm are the smaller of the two bones in both locations.

What's the connection here? This is not clear yet. More study is needed to understand why extra bone growth or deformity occurs in each location (fibula, tibia, ulna, radius) and at which site (medial versus lateral). Exploring these variables might help determine how to predict who will have severe MCE and when and how (and when) to treat it.

Reference: 

Kazuharu Takikawa, MD, et al. Characteristic Factors of Ankle Valgus with Multiple Cartilaginous Exostoses. In Journal of Pediatric Orthopaedics. October/November 2008. Vol. 28. No. 7. Pp. 761-765.

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