Congenital hand differences: embryology and classification.

Congenital hand differences affect approximately 1 in 500 persons and present unique diagnostic and classification challenges. The embryology of the hand and upper extremity is discussed, including the molecular basis for patterning of the upper limb. The Swanson/International Federation of Societies for Surgery of the Hand classification system is described, along with a discussion of the difficulties in classifying congenital hand differences based on morphology or etiology.