Polydactyly of the thumb

**Definition:** A condition caused by development of supernumerary thumbs during the antenatal period.

**Long definition:** Polydactyly of a biphalangeal thumb is the most common form of preaxial polydactyly of fingers, a limb malformation syndrome, that is characterized by the duplication of one or more skeletal components of a biphalangeal thumb. Hands are preferentially affected (in bilateral), and the right hand is more commonly involved than the left.