Dorsal dimelia of a thumb.

Dorsal dimelia (congenital palmar polyonychia) of the hand is defined as the presence of an ectopic palmar nail at the tip of the finger with evidence of dorsalisation of the palmar skin. All reported human cases involved the little or ring fingers and none of the affected digits were subjected to detailed examination. We report the first human case of dorsal dimelia involving a thumb, and provide gross and histological examination of the affected digit. We also review the embryology of this rare deformity and previously reported cases.