The internal organization of the hominoid proximal femur: Interpreting mechanical function from computed tomography and finite element analysis

Historically, researchers have relied for their reconstructions of morphological evolution on the documentation and measurement of surface features. It is suggested here that the internal architecture of long bones provides a novel avenue to function in both extant and fossil animals. The present study outlines the development and application of a method which employs computed tomography (C-T) to compare qualitatively the internal architecture of proximal femora across hominoid taxa. The C-T image data are also used to derive element meshing and material property specifications for finite element models. Each finite element model simulates static loading conditions during one phase of bipedal, quadrupedal, or brachiating locomotion. Modern Homo sapiens, Hylobates lar, Pongo pygmaeus, Pan troglodytes, Gorilla gorilla gorilla, Paranthropus robustus, and European neandertal femora were modelled. Results of the present study suggest that bipedal locomotion in nonhuman hominoids, particularly the Asian apes, engenders stress magnitudes in the femoral shaft that are well in excess of those occurring in the human. Moreover, stresses in the trabecular regions of proximal ape femora are more heterogeneously distributed than those in the trabecular region of the human proximal femur. It is posited here that the internal architectures of African ape femora are adapted to the more broadly disposed stress trajectories associated with postural shifts occurring during multiple locomotor behaviors. In contrast, the internal architecture of the human proximal femur is well suited to the more narrowly disposed force trajectories associated with fully upright bipedal locomotion. Of the fossil hominids, the trabecular organization of Australopithecus africanus resembles that of Homo sapiens, but morphological data were adequate for finite element analysis only in the case of Paranthropus robustus, where maximal stresses in bipedal stances were greater than those found in modern humans, but less than in modern great apes. New technologies often shed unexpected light on longstanding evolutionary problems that are of great significance to our own origins. In the present study, the author considers the utility of combining the qualitative examination of femoral architecture by C-T with computational modelling techniques in providing additional insights into structure and mechanical function in hominid evolution