Identification of tarsal coalition and frequency estimates from skeletal samples

Tarsal coalition is a congenital defect that results when adjacent tarsals fail to separate properly during embryonic development. Anatomically, coalitions present as non-osseous bridges of cartilage or fibrocar- tilage - and occasionally as osseous bridges - between two neighboring bones. In skeletons, non-osseous tarsal coalitions are recognizable as matching lesions between two bones at predictable locations. These coalitions are of interest because they are known to be heritable and are therefore useful for tracing genetic relatives in archaeological cemeteries, because they can be misinterpreted in skeletons as trauma or joint disease, and because they can result in associated pathology. However, despite a considerable literature on tarsal coalition, estimates of coalition frequencies disagree considerably, perhaps due to biases inherent in clinical sampling. In order to gain a better estimate of tarsal coalition frequencies in human populations, data were gathered on 342 European-Americans from the Terry Collection (Smithsonian Institution), 536 South Africans from the Dart Collection (University of Witwatersrand, South Africa), and 756 medieval Danish skeletons (Anthropological Database, Odense University). The Danish skeletons are archaeological, with sample sizes by coalition type ranging from 366-507 individuals. Examples of eight different types of intertarsal coalition were identified among the 1634 skeletons examined. Overall frequency estimates for tarsal coalition ranged from 2.1%-3.5%. South Africans exhibited significantly higher frequencies in the midfoot, with naviculocuneiform I coalition (1.0%) the most common type. Conversely, no coalitions of the midfoot were found among the Euro-Americans or medieval Danes. Instead, these groups exhibited calcaneonavicular coalition as the most common type in the hindfoot (2.0% and 2.1% respectively), while calcaneonavicular coalition was among the least common in the South Africans (0.2%). Copyright 2010 John Wiley & Sons, Ltd.