Comparative analysis of the coxo-femoral joint in hominids in relation to their locomotor behaviour: study of joint correspondence and the degree of congruence between the three-dimensional axes of the two joint components

Abstract

A comparative study of the hip joint in hominids has important implications for physical anthropology, as new insights could provide additional information for documenting changes associated with the acquisition of habitual bipedalism in the human lineage. In the literature, changes in the shape of both the proximal femur and the acetabular region in hominids have been investigated; however, few analyses have studied joint congruence and even fewer have attempted to quantify the degree of correspondence between the three-dimensional orientations of the two joint components. In humans, the hip joint has a lower degree of correspondence in a bipedal posture than would be expected for such a key joint. Because natural selection is dependent on inherited structure and trade-offs between several functions, it has been suggested in a previous study that the lower degree of correspondence might be partly due to the phylogenetic history of our species. In order to test this hypothesis, an extend sample of hominids was analysed. Our results show that joint architecture changes substantially according to the locomotor behaviour of the taxon considered. In orangutans, the considerable mobility of the lower limbs is ensured in part by a shallow acetabulum and a wide femoral neck-shaft angle. In contrast, the hip joint of gorillas has to support heavy loads and thus needs stability: a deep acetabulum and a narrow femoral neck-shaft angle were observed. Finally, our results provided evidence that the highest congruence between the three-dimensional orientations of the two joint components in all the studied genera, i.e. Homo, Pan, Gorilla and Pongo, was obtained in a quadrupedal posture, further suggesting that the shape of the human hip joint is constrained by its quadrupedal ancestry.