Preaxial polydactyly is a congenital malformation, defined by a duplication of a digit on the radial or tibial side. Anatomically, preaxial polydactyly has been classified into different categories; Type I characterized by duplication of one or more of the skeletal components of the first digit. Thumb polydactyly itself is classified into different types, depending on the severity of the duplication. These types range from a mere splitting of the distal phalanx to a completely duplicated thumb. The most common of these types is the duplication of the proximal phalanx resting on a broad metacarpal.
Surgical treatment is the only option available. The aim of surgery is to skeletonize the extra digit and remove it at the metacarpophalangeal (MCP) joint. Post-surgery, the size of the digit is diminished. Therefore, all the soft tissue structure of the duplicated thumb is retained to make the retained thumb look bigger.
Most cases of preaxial polydactyly are sporadic and unilateral. However, several familial cases have been noted. The gene (GLI1; GLI Family Zinc Finger 1) responsible for preaxial polydactyly was mapped to the long arm of chromosome 12.