Thumb polydactyly with symphalangism.

Three cases of thumb polydactyly in which one of the components demonstrated symphalangism are reported. This is a very rare anomaly and only one similar case could be found in the literature. The rarity of this anomaly was explained by the occurrence of two different abnormal molecular events along two different limb growth axes. Finally, the anomaly does not fit into the classification systems described for thumb polydactyly.