Phenotypic analysis of triphalangeal thumb and associated hand malformations.

Triphalangeal thumb (TPT), a long, finger-like thumb with three phalanges instead of two, is regarded as a subtype of preaxial polydactyly. It can occur as a sporadic disorder, but is more often seen as a dominant familial trait. We describe four white Dutch families in which triphalangeal thumb has variable expression and is sometimes associated with preaxial extra rays, rudimentary postaxial polydactyly, cutaneous syndactyly of the hands, and, rarely, postaxial polydactyly and syndactyly of the feet. A comparison with similar familial conditions reported during the past 10 years is provided. The potential significance of linkage and molecular genetic analysis for better insight into the pathogenesis of complex hand malformations is discussed.