has part has part characteristic of inheres in has modifier Abnormality of the proximal phalanx of the thumb Aplasia/Hypoplasia of the proximal phalanx of the thumb 2009-01-29T05:09:41Z HP:0009629 Absent/underdeveloped innermost thumb bone Absent/small innermost thumb bone UMLS:C4024268 This term applies if the proximal phalanx of the thumb is either small/hypoplastic or absent. 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. Aplasia/Hypoplasia of the phalanges of the thumb abnormal aplastic/hypoplastic proximal phalanx of manual digit 1