Radial (thumb) polydactyly occurs in 0.08 per 1000 live births among both black and white populationgroups. It is not as common as ulnar polydactyly. When thumb duplication occurs alone, it is usually unilateral and sporadic. Those involving a triphalangeal thumb may be inherited as an autosomal dominant trait. As with triphalangeal thumb, delta phalanx has often been found to have a strong family history of inheritance. The basic classification system was defined by Wassel in 1969. His classification was based on the level of duplication and the number of bones in the thumb, and has seven groups (types I to VII) according to the level of the bifurcation. The thumb is often hypoplastic (smaller than normal), so the reconstructed thumb is nearly always smaller than the opposite side.
Classification--Types I to VII based on level of duplications:
III: bifid proximal phalanx (PP) (digit bone nearest the palm)
IV: most common type with duplication of proximal phalanx which rest on broad metacarpal
V: bifid metacarpal (MC) (bone in palm)
VI: duplicated MC
Treatment of Type IV should be done very early, because if one waits too long, the supernumerary component displaces the normal component into markedly radial or ulnar deviation. If left, it will continue to grow this way.
Monozygotic Twins Disordant for Thumb Polydactyly; Plastic & Reconstructive Surgery. Vol 113(1):449-451, January 2004; Peterson, Steven L. D.V.M., M.D.; Rayan, Ghazi M. M.D.
Triplicated Thumb; Plastic & Reconstructive Surgery. Vol 100(2):418-421, August 1997; Atabay, Kenan M.D.; Latifoglu, Osman M.D.; Demirkan, Ferit M.D.; Yavuzer, Reha M.D.
Treatment of Congenital Upper Extremity Problems; Plastic & Reconstructive Surgery. Vol 119(5):101e-129e, April 15, 2007; Netscher, David T. M.D.; Baumholtz, Michael A. M.D.
Thumb Polydactyly: Clinical Outcome after Reconstruction; CH Yen, WL Chan, HB Leung, KH Mak; Journal of Orthopaedic Surgery 2006;14(3):295-302