Causes / typical injury mechanism: Polydactyly may be due to an isolated trait or as part of a genetic syndrome. Multiple gene loci have been linked to the development of polydactyly, including:

• GLI3 (associated with Greig cephalopolysyndactyly syndrome)
• SHH
• FGF
• ALX4
• 5'HoxD
• ZNF141
• MIPOL1
• IQCE
• PITX1

Classic history and presentation: Polydactyly presents in infancy, or patients may present later in childhood when shoe fitting becomes a concern. It may appear in isolation or with other birth defects.

Prevalence:

• Represents 45% of congenital foot anomalies.
• Incidence of 1.7 per 1000 live births.
• No sex predilection.
• Postaxial – Most common, constituting 80% of duplications.
• Preaxial – 15% of duplications.
• Central – 5% of duplications.

Risk factors:

• About 30% of patients with isolated polydactyly have a positive family history.
• Syndromes that feature polydactyly include Ellis-van Creveld syndrome, trisomy 13 syndrome, tibial hemimelia, trisomy 21 (Down syndrome), and Greig cephalopolysyndactyly syndrome.
• Patients of African descent have a higher incidence rate of 3.6-13.9 per 1000 live births.

Pathophysiology: Two theories may explain the development of polydactyly: a disorder in the programmed cell death during fetal limb development, or the genetic mutations of some gene loci.

Grade / classification system:

• Classified according to its location as preaxial (first ray), central (middle rays), or postaxial (fifth ray).
• Temtamy and McKusick classification – This classifies cases as preaxial, postaxial, or complicated and is used by geneticists.
• Venn-Watson (V-W) classification – Based on the shape of the associated metatarsal bone.
  • Normal metatarsal with distal duplication
  • Block metatarsal
  • Y-shaped metatarsal
  • T-shaped metatarsal
  • Normal metatarsal shaft with a wide head
  • Ray duplication
• Watanabe-Fujita classification of medial ray polydactyly.
  • Tarsal
  • Metatarsal
  • Proximal phalangeal
  • Distal phalangeal