Os intermetatarseum: a heritable accessory bone of the human foot.

The os intermetatarseum is a relatively common accessory bone of the human foot located at the tarsometatarsal border between the first and second metatarsals. It can occur as an independent ossicle or as an osseous spur projecting from the proximal ends of the first two metatarsals, or the distal end of the first cuneiform. To determine the frequency of this congenital defect in native North American groups and East Asians from Japan, the skeletons of 846 Native Americans and 125 modern Japanese and Ainu were examined for the presence of the os intermetatarseum. The North American skeletons are from archaeological sites in various parts of Canada and the United States, including the Arctic coast, the Subarctic, the Aleutian Islands, the Northern Plains, the Illinois River area, and the Southwest. Overall frequencies ranged from no occurrence among the Arctic samples to 8% of individuals from Pecos Pueblo. Second metatarsal spurs occurred in particularly high frequencies among American Indians, whereas the East Asians were only found to have os intermetatarseums associated with the first cuneiform. Because pedigrees have shown the os intermetatarseum to be an inherited defect, its high frequency among some Native Americans may be due, in part, to a higher degree of genetic relatedness among the individuals in the North American samples than among the relatively modern East Asians.