has part has part characteristic of inheres in has modifier Aplasia/Hypoplasia of the proximal phalanx of the thumb Absent proximal phalanx of thumb 2009-01-29T05:11:02Z HP:0009637 UMLS:C4021418 HP:0004077 Absent ossification/absent proximal thumb phalanx Absent innermost thumb bone Absence of the proximal phalanx of the thumb. 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 of the proximal phalanx of the thumb Partial absence of thumb Aplasia of the middle phalanx of the hand abnormal aplastic proximal phalanx of manual digit 1