Variation in mammalian proximal femoral development: comparative analysis of two distinct ossification patterns

The developmental anatomy of the proximal femur is complex. In some mammals, including humans, the femoral head and greater trochanter emerge as separate ossification centres within a common chondroepiphysis and remain separate throughout ontogeny. In other species, these secondary centres coalesce within the chondroepiphysis to form a single osseous epiphysis much like the proximal humerus. These differences in femoral ontogeny have not been previously addressed, yet are critical to an understanding of femoral mineralization and architecture across a wide range of mammals and may have key implications for understanding and treating hip abnormalities in humans. We evaluated femora from 70 mammalian species and categorized each according to the presence of a 'separate' or 'coalesced' proximal epiphysis based on visual assessment. We found that ossification type varies widely among mammals: taxa in the 'coalesced' group include marsupials, artiodactyls, perissodactyls, bats, carnivores and several primates, while the 'separate' group includes hominoids, many rodents, tree shrews and several marine species. There was no clear relationship to body size, phylogeny or locomotion, but qualitative and quantitative differences between the groups suggest that ossification type may be primarily an artefact of femoral shape and neck length. As some osseous abnormalities of the human hip appear to mimic the normal morphology of species with coalesced epiphyses, these results may provide insight into the aetiology and treatment of human hip disorders such as femoroacetabular impingement and early-onset osteoarthritis.