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).