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Little is known about thenar dysplasia in radial polydactyly, other than that thenar hypoplasia occasionally occurs in radial polydactyly with triphalangism. In particular, the phenotype and level of duplication associated with thenar dysplasia remain unclear.The abductor pollicis brevis and flexor pollicis brevis muscles were visualized using three-dimensional ultrasound, and their horizontal geometry was assessed using a biaxial level classification system. Subjects were categorized into three phenotypes according to the developmental condition of the radial thumb. The relationship between the level of distribution of the muscles and the level of the bifurcation of the radial thumb was investigated.Nineteen patients with radial polydactyly without triphalangism were included. There were 10 patients with the nonfloating type, three with the floating type, and six with the rudimentary type. All patients with bifurcation at or more distal to the metacarpophalangeal joint had normal thenar muscle distribution, but the muscles in patients with bifurcation at or more proximal to the level of the metacarpals were confined, regardless of phenotype. The level of muscle distribution was strongly correlated with the level of the bifurcation of the radial thumb.These findings suggest that formation of the thenar muscles in the longitudinal direction in radial polydactyly might depend on the level of bifurcation of the radial thumb. The presence of thenar dysplasia even in floating-type or rudimentary-type duplications is of clinical and etiologic importance.