Complete duplication of the proximal phalanx of the thumb. On x-ray two separate bones appear side to side. In contrast to the proximal phalanges of the digits 2-5, the proximal phalanx of the thumb is embryologically equivalent to the middle phalanges of the other digits, whereas the first metacarpal is embryologically of phalangeal origin and as such equivalent to the proximal phalanges of the other digits. The phenotype is Human Phenotype Ontology entry HP_0009608 (Complete duplication of proximal phalanx of the thumb). Also known as: Complete duplication of the innermost bone of the thumb.