The Variablity of Cortical Bone Stiffness along the Femur Shaft

Cortical bone is a calcified connective tissue that comprises the rigid outer portion of bones. For this study, we examined the stiffness of this material in its three orthogonal directions (axial, radial, and transverse) in three locations along the femurs of white-tailed deer (proximal, mid-diaphysis, and distal). Based on Wolff’s law of bone adaptation, we would expect to observe higher values of stiffness in areas that experience the most loading (e.g., mid-diaphysis, axial direction; assuming the bone is loaded in bending). Therefore, our working hypotheses for this study were that (1) bone samples will have the highest stiffness when loaded in the axial direction (the direction of physiological loading), and that (2) mid-diaphyseal samples would have the highest stiffness when compared to samples from the proximal and distal femur. To test these hypotheses, ninety cubical samples (2 mm3 in volume) were prepared using a low-speed diamond saw. These cubes were then tested in compression using an Instron 5942 universal testing machine. Tests were repeated a total of three times for each orthogonal direction. Stiffness values were significantly higher in the axial direction (compared to the radial and transverse directions) in all locations (proximal, mid-diaphysis and distal femur), thus supporting our first hypothesis. These results are consistent with Wolff’s Law, since the axial direction, which normally experiences the most loading in the femur, showed the highest stiffness values. Our results also demonstrated a statistically significant difference in the radial and transverse directions for stiffness of the mid-diaphysis (but not the axial direction), thus refuting our second hypothesis. These similar stiffness values in the axial direction may imply that, due to muscle action, the femur is not loaded in bending but almost pure compression