Cortical bone distribution in the femoral neck of strepsirhine primates

The thickness of the inferior and superior cortices of the femoral neck was measured on X-rays of 181 strepsirhine primate femora representing 24 spec ies. Neck length, neck depth and neck-shaft angle were also measured. The s trength of the femoral neck in frontal bending was estimated by modeling th e neck as a hollow cylinder, with neck depth as the outer diameter and cort ical thickness representing the superior and inferior shell dimensions. Res ults indicate that the inferior cortex is always thicker than the superior cortex. The ratio of superior to inferior cortical thickness is highly vari able but distinguishes two of the three locomotor groups in the sample. Ver tical clingers and leapers have higher ratios (i.e., a more even distributi on of cortical bone) than quadrupeds. The slow climbers tend to have the lo west ratios, although they do not differ significantly from the leapers and quadrupeds. These results do nor confirm prior theoretical expectations an d 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 leng th, and neck-shaft angle, calling into question whether the short neck of s trepsirhine primates acts as a cantilever beam in bending. On the other han d, the estimated section moduli are highly correlated with body mass and ne ck length, a correlation that is driven primarily by body mass. In conclusi on, we believe that an alternative interpretation to the cantilever beam mo del 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 s tudies of cross-sectional geometry of long bones, we found slightly positiv e allometry of cortical dimensions with body mass. (C) 2000 Academic Press.