Thumb Polydactyly

► [Duplication of the thumb]

As stated earlier, preaxial Polydactyly is a heterogeneous category falling into at least four types: (1) thumb polydactyly; (2) Polydactyly of triphalangeal thumb; (3) polydactyly of index finger; and (4) poly-syndactyly. Polydactyly of triphalangeal thumb is discussed in the previous section, while polydactyly of the index finger and polysyndactyly are discussed in the section "Preaxial Polydactyly."

'Thumb Polydactyly (polydactyly preaxial I, OMIM 174400) is the term used to refer to duplication of one or more of the skeletal components of a biphalangeal thumb. Thumb polydactyly has been further classified into six types, depending on whether the distal phalanx, proximal phalanx, and metacarpal are bifid or fully duplicated (Fig. 6.61 a-c). The most common type is a duplicated proximal phalanx resting on a broad metacarpal (Fig. 6.62). The Fromont anomaly, i.e., hypoplasia or aplasia of the thumb musculature, can be regarded as a minor expression of thumb polydactyly that is inherited as an autosomal dominant trait (Graham et al. 1987). Thumb polydactyly is a common form of polydactyly in many populations (Handforth 1950) and is 3-4 times as frequent in American Indians as in Caucasians (Bingle and Niswander 1975), with a higher frequency in females than in males. As in all polydactylies, thumb duplication is rarely (20% of cases) associated with other congenital anomalies, except in recognizable syndromes (Castilla et al. 1998). Unlike postaxial polydactyly type B (skin tag extra-digit), which is bilateral in about 50% of cases, preaxial polydactyly is almost always unilateral (Bingle and Niswander 1975). The genetics of thumb polydactyly is not clear. Either a single autosomal dominant gene with reduced penetrance or a multi-genic threshold model might be involved. To investigate whether thumb and hallux duplication have different clinical and epidemiological characteristics, Orioli's group studied 920 newborn infants with 1st digit duplication, who were ascertained among 3,444,374 births by the Latin-American Collaborative Study of Congenital Malformations from 1967 to 1995 (Orioli and Castilla 1999). Since biphalangeal thumb duplication or hallux duplication can occur in

Frequency Polydactyly America

Fig. 6.61 a-c. Thumb polydactyly.a In a 23-year-old man.Note syndactyly. The metacarpal is normal. Observe also mesopha-

duplication of the distal phalanx of the thumb, bifid proximal langism with clinodactyly of the 5th finger. The other hand phalanx, and normal metacarpal. Unilateral, isolated defect. was unaffected. c Stillborn female with VATER association.

b In a 10-month-old girl. Complete duplication of the proximal Note duplication of the distal and proximal phalanx of the and distal phalanges of the thumb, combined with soft tissue thumb and forked 1st metacarpal

Fig. 6.61 a-c. Thumb polydactyly.a In a 23-year-old man.Note syndactyly. The metacarpal is normal. Observe also mesopha-

duplication of the distal phalanx of the thumb, bifid proximal langism with clinodactyly of the 5th finger. The other hand phalanx, and normal metacarpal. Unilateral, isolated defect. was unaffected. c Stillborn female with VATER association.

b In a 10-month-old girl. Complete duplication of the proximal Note duplication of the distal and proximal phalanx of the and distal phalanges of the thumb, combined with soft tissue thumb and forked 1st metacarpal

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