Structural design of the femoral neck in primates.

This study investigates the structural design of the femoral neck with respect to phylogeny, locomotor behavior, and body size in a large comparative sample of primates. Since a bone's strength is determined by the amount and arrangement of its constituent material, the amount and distribution of bone in the femoral neck are the focus of the present study. Two types of data were collected. First, the total distribution of bone across the femoral neck (perpendicular to the femoral neck axis) was examined using image analysis techniques. The distribution of both cortical and trabecular bone are included in this analysis. Secondly, the external dimensions of the femoral neck and the thickness of the superior and inferior cortices, measured from radiographs, were used to calculate femoral neck cross-sectional properties (cortical area and second moment of area). These data were input into a simplified cantilevered beam model that incorporates femoral neck-shaft angle and femoral neck length and is used to predict stress under one unit body mass of loading on the femoral head. The patterns of bone distribution in the femoral neck between taxa are broadly similar for the analysis that includes both trabecular and cortical bone and that which includes only cortical bone. In all cases, there tends to be more bone on the inferior aspect of the femoral neck compared to the superior aspect. However, the non-human hominoids and New World monkeys (e.g., Ateles and Alouatta) have a more even distribution of bone across the femoral neck and have thicker superior cortices relative to inferior cortices compared to the rest of the comparative sample, including humans. It is proposed that the more equal distributional pattern in the non-human hominoids and atelines reflects less stereotypical, more generalized loading orientations. Differences between species in overall strength of the femoral neck are a function of both body size and locomotor mode.