Abnormal 1st metacarpal morphology Triangular shaped 1st metacarpal 2009-05-27T04:24:30Z UMLS:C4024092 In contrast to the metacarpals 2-5, the first metacarpal is embryologically of phalangeal origin and as such equivalent to the proximal phalanges of the digits 2-5, whereas the proximal phalanx of the thumb is equivalent to the middle phalanges of the other digits. A triangular or so called delta shaped phalanx is a typical result after a bracket epiphysis of the affected phalanx (in this case metacarpal, see explanation above). This term applies to a triangular shaped 1st metacarpal. HP:0010033 Triangular shaped 1st long bone of hand