Biphalangeal and triphalangeal toes in the evolution of the human foot.

The number of phalanges of the human toes was investigated in a series of 2,550 radiographs. Classical triphalangia of the lateral toes (2-5) was observed in 1,440 cases (56.47%). Biphalangeal disposition was observed for the 5th toe in 1,110 cases (41.02%), for the 4th toe in 64 cases (2.51%), for the 3rd toe in 5 cases (0.20%), and for the 2nd toe in 3 cases (0.12%). The frequency of biphalangia of a given toe was not independent of the others. Biphalangeal toes result primarily from the absence of development of the distal interphalangeal joint. Biphalangia of the toes is a derived character which is restricted, within primates, to the human species, in relation to the reduction of the toes in adaptation to bipedalism.