36.1.1 Radial Polydactyly

Radial polydactyly has an incidence of 1 in every 3,000 births. Variable degrees of duplication exist and range from widening of the thumb tip to complete duplication. The most common classification of thumb duplication has been described by Wassel, with type IV representing the most frequent pattern. Types I, III, and IV refer to bifid phalanges, while types II, IV, and VI refer to complete phalangeal duplication. Wassel type VII describes a thumb with an extra, or third, phalanx, also known as a triphalangeal thumb (Fig. 36‑1). Radial polydactyly with triphalangism is associated with an autosomal-dominant inheritance pattern and genetic mutations within the hedgehog pathway. The remaining types of preaxial polydactyly are thought to be the result of spontaneous mutations. Radial polydactyly often occurs in isolation but may be associated with Fanconi anemia or Holt–Oram syndrome.

36.1.2 Central Polydactyly

Central polydactyly (duplication of the index, middle, or ring fingers) is often seen in combination with other anomalies, most commonly syndactyly, known as synpolydactyly. This is felt to be a heritable condition with a broad spectrum of phenotypic variability. Stelling and Turek have classified central polydactyly into three main types: (1) extra soft-tissue mass only, (2) duplication of a digit or partial digit that articulates with an enlarged or bifid phalanx or metacarpal, and (3) duplication of a digit including the metacarpal and all soft tissues involved (Fig. 36‑2). In 2015, Wall et al reviewed 56 hands in 40 patients to establish a radiographic classification for synpolydactyly. The system is based on level of duplication with type I representing metacarpal duplication and type III representing middle or distal phalanx duplication (Fig. 36‑3).

36.1.3 Ulnar Polydactyly

The original ulnar polydactyly classification by Stelling and Turek was a three-part classification system similar to that described earlier for central polydactyly. More commonly, ulnar polydactyly is classified into types A and B as described by Temtamy and McKusick. Type A represents a well-formed extra digit that articulates with the small finger metacarpal or a duplicate small finger metacarpal (Fig. 36‑4). Type B ulnar polydactyly describes a rudimentary digit with only soft-tissue attachments to the small finger (Fig. 36‑5). Type B can present as a spectrum ranging from a small nubbin on the ulnar border of the digit to a relatively well-formed digit with only soft-tissue attachment and no articulation. The presence of a nubbin is thought to represent a remnant or neuroma of a pedunculated supernumerary digit that autoamputated in utero. Pritsch et al have proposed a subclassification of type A ulnar polydactyly: type 1, fully developed sixth metacarpal; type 2, intercalated supernumerary digit between ring and small finger with rudimentary metacarpal; type 3, supernumerary digit ulnar to small finger with rudimentary metacarpal; type 4, digit articulates with fifth metacarpophalangeal joint; and type 5, supernumerary digit arises from a bifid proximal phalanx. Patients with type A ulnar polydactyly present with bilateral hand involvement quite commonly (69%) and also frequently have foot involvement. The majority of type B ulnar polydactyly is bilateral.

Type B postaxial polydactyly is uncommon in Caucasian infants, but relatively common in African American infants, with respective prevalence of approximately 1 in 1,300 and 1 in 150 live births. Type A is relatively rare and has been reported as equal between African American and Caucasian patients, but a recent review demonstrated increased relative frequency among Caucasians and Hispanic patients compared with African American patients. Isolated, nonsyndromic ulnar polydactyly, inheritance is more commonly an autosomal-dominant pattern, but recessive patterns of inheritance have also been described. Associated congenital anomalies and syndromes can also be present with ulnar polydactyly including Ellis–van Creveld, Smith–Lemhi–Optic, McKusick–Kaufmann syndromes, short-rib polydactyly syndrome I, orofaciodigital syndrome III, Bardet–Biedl syndrome, Meckel–Gruber syndrome, Greig cephalopolysyndactyly, and Pallister–Hall syndrome.