Preaxial polydactyly of fingers is a limb malformation syndrome characterized by the attachment of a superfluous digit on the first digit. Four types have been defined: Type I (PPD1 or biphalangeal thumb polydactyly) which shows duplication of one or more skeletal components of a biphalangeal thumb; type II (PPD2 or polydactyly of a triphalangeal thumb) which involves the presence of a usually opposable triphalangeal thumb with or without additional duplication of thumb; type III (PPD3 or polydactyly of an index finger) where the thumb is replaced by one or two triphalangeal digits with dermatoglyphic pattern specific for the index finger; and type IV (PPD4 or polysyndactyly) which shows variably mild degrees of thumb duplication and variable syndactyly between 3rd and 4th fingers. Among the four types, PPD1 is the most frequent form. Preaxial polydactyly of fingers is caused by disruptions to the developmental patterning of the limb along the anterior-posterior axis that lead to changes in digit number and identity.
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