Causes / typical injury mechanism:

• Polydactyly of the hand is inherited. This may be a sporadic genetic mutation or may have an autosomal dominant inheritance pattern with variable penetrance.
• Sonic hedgehog (SHH) / zone of polarizing activity pathways regulate anterior / posterior (ulnoradial) development.
• Currently 6 genes have been identified as contributing to polydactyly of the hand: ZNF141, GLI3, MIPOL1, IQCE, PITX1, and GLI1.

Classic history and presentation: Polydactyly is present at birth with various degrees of maturity of the accessory digit(s).

Prevalence: There is a male prevalence for both preaxial (thumb duplication) and postaxial (ulnar digit) polydactyly.

Risk factors:

• Preaxial polydactyly has a higher prevalence in White individuals.
• Postaxial polydactyly has a 10 times higher prevalence in Black individuals with an autosomal dominant inheritance pattern. White patients with postaxial polydactyly need genetic work-ups.

Pathophysiology:

• The limb bud forms from ectoderm with underlying mesoderm. Limb development takes place in weeks 4-8 of gestation.
• The zone of polarizing activity is an area of mesenchyme beneath the apical ectodermal ridge on the posterior aspect of the limb bud.
• Via secretion of SHH, there is development of the limb in the ulnoradial dimension.
• Oversecretion of SHH results in postaxial polydactyly. Ectopic SHH expression or disruption of this signaling pathway leads to preaxial polydactyly.

Grade / classification system:

• Postaxial polydactyly –
  • Type A: well-formed digit
  • Type B: rudimentary digit
• Preaxial polydactyly (thumb duplication) – Wassel classification
  • Type I: bifid distal phalanx
  • Type II: duplicated distal phalanx
  • Type III: bifid proximal phalanx
  • Type IV (most common): duplicated proximal phalanx
  • Type V: bifid metacarpal
  • Type VI: duplicated metacarpal
  • Type VII: triphalangism