Polydactyly is the presence of extra digits. Its incidence is likely underestimated because many practitioners treat simple “nubbins” without referring them to orthopedic specialists.1-3 Polydactyly can be detected by ultrasound as early as 14 weeks’ gestational age, with partial autoamputation seen in most isolated polydactylies.4 The thumb, responsible for 40% of hand function, must be able to oppose the other digits with a stable pinch.5 Polydactyly encumbers this motion when the duplicated digits deviate from normal alignment. Ezaki6 noted that the anatomy is better described as “split” than “duplicated.” There are many dichotomous ways to classify polydactyly: preaxial (radial) versus postaxial (ulnar), thumb versus triphalangeal, simple versus complex (Figure 1). Mixed polydactyly is defined as the presence of preaxial and postaxial polydactyly.7 Surgical management seeks to allow normal hand function and to restore cosmesis.
Epidemiology
Sun and colleagues8 reported the overall polydactyly incidence as 2 per 1000 live births in China from 1998 to 2009, with a slight male predominance; polydactyly was also 3 times more common than syndactyly in this population. Ivy,9 in a 5-year audit of Pennsylvania Department of Health records, found polydactyly to be the fourth most common congenital anomaly after clubfoot, cleft lip/palate, and spina bifida. Thumb duplication occurs in 0.08 to 1.4 per 1000 live births and is more common in American Indians and Asians than in other races.5,10 It occurs in a male-to-female ratio of 2.5 to 1 and is most often unilateral.5 Postaxial polydactyly is predominant in black infants; it is most often inherited in an autosomal dominant fashion, if isolated, or in an autosomal recessive pattern, if syndromic.1 A prospective San Diego study of 11,161 newborns found postaxial type B polydactyly in 1 per 531 live births (1 per 143 black infants, 1 per 1339 white infants); 76% of cases were bilateral, and 86% had a positive family history.3 In patients of non-African descent, it is associated with anomalies in other organs. Central duplication is rare and often autosomal dominant.5,10
Genetics and Development
As early as 1896, the heritability of polydactyly was noted.11 As of 2010, polydactyly has been associated with 310 diseases.12 Ninety-nine genes, most involved in regulation of anterior-posterior formation of the limb bud, have been implicated.12,13
The upper limb begins to form at day 26 in utero.14 Apoptosis in the interdigital necrotic zones results in the formation of individual digits. It is presumed that, in polydactyly, the involved tissue is hypoplastic because of an abnormal interaction between mesoderm and ectoderm.5 Presence of an apical ectodermal ridge determines the formation of a limb bud, and on it the zone of polarizing activity (ZPA) dictates preaxial and postaxial alignment.14,15 The ZPA is located on the posterior zone of the developing limb bud. The levels of GLI3, a zinc finger-containing DNA-binding protein, are highest in the anterior area, and HAND2, a basic helix-loop-helix DNA-binding protein, is found in the ZPA. This polarity promotes sonic hedgehog (Shh) gene expression in the posterior region, which in turn prevents GLI3 cleavage into its repressed form. GLI3R (repressed) and GLI3A (active) concentrations are highest, therefore, in the anterior and posterior portions of the bud, respectively. The GLI3A:GLI3R ratio is responsible for the identity and number of digits in the hand (ie, the thumb develops in regions of high GLI3R). GLI and Shh mutations lead to polydactylous hands with absent thumbs (Figure 2).16
Ciliopathies have also been shown to cause postaxial polydactyly, possibly because of the role that nonmotile cilia play in hedgehog signaling.17 Mutations in Shh genomic regulators cause preaxial polydactyly.18 HoxD activates Shh in the ZPA; HoxD13 mutations are associated with synpolydactyly.16,19 In each of these mutations, Shh production is altered, and some form of polydactyly results.
Associations
Many syndromes have been associated with polydactyly. Not all polydactyly is associated with other disorders, but the more complex the polydactyly, the more likely that other anomalies are present. Every patient who presents with polydactyly should have a full history taken and a physical examination performed (Figure 3). Any patient with syndromic findings or atypical presentations (eg, triphalangism, postaxial polydactyly in a patient of non-African descent, central and index polydactyly) should be referred to a geneticist.
Classifications
The Wassel20 classification describes the anatomical presentation of thumb duplication on the basis of 70 cases in Iowa (Figures 4, 5; Table 1). Because some duplications fall outside the Wassel classification, many researchers have proposed modifications (Figure 6).21-25
