Complete duplication of proximal phalanx of the thumb 2009-01-29T03:32:28Z Complete duplication of the innermost bone of the thumb HP:0009608 UMLS:C4024273 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. HP:0004078 Complete duplication of thumb phalanx