Classification of polydactyly of the hands and feet

The authors present a new classification of polydactyly based on radiomorphological alterations. The malformations are defined in two directions, as in a system of coordinates. The longitudinal arrangement is based on the pathogenetic principle of bifurcation of a finger or a toe ray from distal to proximal. We accordingly divided polydactylies into five types: distal phalanx, middle phalanx, proximal phalanx, metacarpal or metatarsal, carpal or tarsal. The transverse arrangement indicates which rays are involved. All polydactylies, including the special forms such as rudimentary manifestations, triphalangism, and multiple duplications, can be incorporated into this simple basic scheme. Depending on their characteristics, the special forms are further subdivided, e.g., into a distal or proximal phalanx type with simple or double triphalangism, or a tarsal type with third-degree duplication and first-degree aplasia. Numerous radiological examples and schematic drawings illustrate the classification. The advantage of the classification is that it depends exclusively upon the skeletal finding and all manifestations are registered according to a simple scheme longitudinally and transversely. This makes them codifiable for the computer and suitable for multicenter studies. The special forms, the rudiment, triphalangism, and multiple duplication, can easily be further subclassified. Moreover, the nomenclature is simple and is oriented to anatomical terminology.