Central polydactyly has been classified by the International Federation of Societies for Surgery of the Hand1 as a type III polydactyly. The Japanese Society for Surgery of the Hand has proposed this malformation be classified with cleft hands and osseous syndactyly under a new category of failure of ray induction.2 This is based on experimental work with embryonic rats treated with a chemotherapeutic agent (busulfan) in which the tetrogenic effects resulting in cleft hands, central polydactylies, and osseous syndactylies were all from defects in the central hand plate due to failure of induction.3 The experimental data indicate there is diffuse cell death in the central ectoderm and mesoderm with decreased expression of fibroblast growth factor in the apical ectodermal ridge as well as bone morphogenetic protein-4 and sonic hedgehog in the mesoderm. A similar pattern of interdigital apoptosis and cartilage condensation leads to abnormal induction of digital rays in cleft hands, central polydactylies, and osseous syndactylies.4^,5

Central synpolydactyly was one of the first two limb malformations identified as being caused by a genetic mutation in a homeobox (HOX) gene.6 Specifically, it is deletions in HOXD13 on chromosome 2 (2q31) that result in this malformation. It is an autosomal dominant inheritance pattern.7 There is a spectrum of presentation in each family but generally with marked osseous deformity (Figure 5-2).8

Central polydactyly by definition involves the ring, long, and/or index fingers. It is much less common than postaxial polydactyly or preaxial polydactyly. It occurs most often in the ring > long > index fingers. Bilateral involvement is typical, and associated foot anomalies are frequent. Partial duplications can occur and appear similar to those partial polydactylies of the thumb and small finger by both clinical exam and radiographs. Independent, mobile complete polydactylies also occur. At times, it is unclear which central digit is duplicated. In the presence of syndactyly, the numbers of bones and their alignment can be hard to assess by clinical exam. Radiographs are necessary to determine the presence of bracketed epiphyses, crossed bones, bifid bones, synostoses, joint malalignment, and physeal growth potential. The osseous abnormalities may extend into the palm. Radiographs are mandatory to decide on surgical intervention and planning.

Central polydactyly is associated with Grebe chondrodysplasia and syndrome C (trigonocephaly). Central synpolydactyly is an autosomal dominant inherited mutation of the HOXD13 gene on chromosome 2. Children of consanguineous pregnancies have a more severe form of the osseous disorder that extends into the metacarpals and carpus, metatarsals and tarsal bones.7

It is what you learn after you know it all that counts. —Earl Weaver