Cortical bone distribution in the femoral neck of strepsirhine primates

Abstract

The thickness of the inferior and superior cortices of the femoral neck was measured on X-rays of 181 strepsirhine primate femora representing 24 species. Neck length, neck depth and neck-shaft angle were also measured. The strength of the femoral neck in frontal bending was estimated by modeling the neck as a hollow cylinder, with neck depth as the outer diameter and cortical thickness representing the superior and inferior shell dimensions. Results indicate that the inferior cortex is always thicker than the superior cortex. The ratio of superior to inferior cortical thickness is highly variable but distinguishes two of the three locomotor groups in the sample. Vertical clingers and leapers have higher ratios (i.e., a more even distribution of cortical bone) than quadrupeds. The slow climbers tend to have the lowest ratios, although they do not differ significantly from the leapers and quadrupeds. These results do not confirm prior theoretical expectations and reported data for anthropoid primates that link greater asymmetry of the cortical shell to more stereotypical hip excursions. The ratio of superior to inferior cortical thickness is unrelated to body mass, femoral neck length, and neck-shaft angle, calling into question whether the short neck of strepsirhine primates acts as a cantilever beam in bending. On the other hand, the estimated section moduli are highly correlated with body mass and neck length, a correlation that is driven primarily by body mass. In conclusion, we believe that an alternative interpretation to the cantilever beam model is needed to explain the asymmetry in bone distribution in the femoral neck, at least in strepsirhine primates (e.g., a thicker inferior cortex is required to reinforce the strongly curved inferior surface). As in prior studies of cross-sectional geometry of long bones, we found slightly positive allometry of cortical dimensions with body mass.