This term applies if the epiphysis of the proximal phalanx of the thumb, which is located at the proximal end of the phalanx, does not appear in concordance with gender and age dependant norms as seen on x-rays. 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. [from
HPO]