Polydactyly of the thumb: a modification of the Wassel-Flatt classification.

Although the Wassel-Flatt classification system has been widely used for radial polydactyly, it has some limitations. We modified the classification system by introducing the hypoplastic types and refining the definition of triphalangeal thumb without changing the main structure of the original classification system. A total of 200 consecutive duplicated thumbs of 183 patients treated surgically from June 2016 to June 2018 were included. We evaluated intra-observer and inter-observer reliability using the kappa coefficient in the modified and original Wassel-Flatt classification systems (three examiners evaluated each case twice, with an interval of 4 weeks). We also evaluated the surgical methods according to the types of deformity in the modified and original Wassel-Flatt classification systems. The modified Wassel-Flatt classification system had good inter-observer reliability and provides useful information for determining the surgical plan according to the types of radial polydactyly.Level of evidence: IV.