Postaxial Polydactyly Type B
Postaxial polydactyly type B (PAP-B) features a nonfunctional, partially formed, ulnar (or fibular) digit with no bony attachments, such as the ones seen on the hands of this patient.
In postaxial polydactyly type A, the supernumerary digit is fully formed and articulates with the fifth (or additional) metacarpal or metatarsal.
PAP-B has an autosomal dominant pattern of inheritance and more commonly affects people of African descent than those in the white population. In the United States, the reported frequency of PAP-B in African Americans is 1 in 143 live births, compared with 1 in 1,339 live births in the white population.1
The diagnosis is largely clinical, but radiographs usually are recommended to ensure that there is no bony involvement prior to the application of surgical clips, which is the definitive treatment.2
PAP-B can be associated with various genetic syndromes, but it often is a normal variant.n
References
1. Watson BT, Hennrikus WL. Postaxial type-B polydactyly: prevalence and treatment. J Bone Joint Surg Am. 1997;79(1):65-68.
2. Mills JK, Ezaki M, Oishi SN. Ulnar polydactyly: long-term outcomes and cost-effectiveness of surgical clip application in the newborn. Clin Pediatr (Phila). 2014;53(5):470-473.