Triphalangeal thumb.

Triphalangeal thumb (TPT), a rare malformation of uncertain pathogenesis, may occur as an isolated defect, in association with other malformations of the hands, or as a feature of a syndrome or sequence. Isolated TPT occurs in two functional types: opposable and non-opposable. The latter appears to be inherited as a simple autosomal dominant trait, while the former is generally sporadic. TPT is associated with a number of specific malformations of the hand or foot, several of which have a well documented autosomal dominant pattern of inheritance. TPT is a feature of a number of specific syndromes. In this setting it may be associated with radial hypoplasia, bone marrow dysfunction, congenital heart disease, lung hypoplasia or agenesis, anorectal malformations, sensorineural hearing loss, onychodystrophy, mental retardation, and other disorders. TPT serves as a useful marker in such patients; in conjunction with the clinical and radiological findings, it can help to establish the correct diagnosis, leading to appropriate management and genetic counselling.