Ulnar dimelia--a limb without anteroposterior differentiation.

A case of ulnar dimelia with radial aplasia and polydactyly and the main features of its treatment are described. I believe that this abnormality results from impairment of the mechanisms responsible for organization and differentiation of the limb bud in the anteroposterior direction. This deformity is not the result of duplication of any of the parts of the limb, either compensatory or because of a primary duplicated limb field. The anomaly should be called "anteroposteriorly undifferentiated limb with polydactylism" because the long bones in the forearm are probably not ulnae. Similarities exist between the human condition and certain mutations that appear from time to time in chickens, in which a primary mesodermal defect has been found to be at fault.