Cortical Bone Stiffness in the Axial Direction Demonstrates the Highest Variance Between Samples

Bone is a biomaterial; therefore, its mechanical properties start to deteriorate the moment it is removed from the living organism. When tested in the lab, bone samples are usually harvested and then fresh frozen until one day prior to the experiment, when they are thawed in the refrigerator for 24 hours. Here, we investigated the effects of much longer thawing times (7 days) on bone stiffness. Thirty cortical bone cubes were harvested from the proximal diaphysis of 5 young white-tailed deer femora and then were frozen (-20 °C). Next, the cubes were tested in compression in the axial, transverse and radial directions, both one day and 7 days after they were thawed and kept in the refrigerator (4 °C). Bone stiffness decreased by 5 % (not significant) when results were compared between day one and seven. Yet stiffness variance (measured as standard deviation, SD) was significantly different between the 3 directions, both on day one and seven. Axial stiffness showed the highest variance among samples, followed by transverse and radial stiffness. Wolff’s law predicts that bone will adjust itself to loading (modifying bone mass and orientation). Thus, it predicts that the highest variance in stiffness will be in the direction which is normally loaded (i.e., this direction will be the most susceptible to changes due to the individual’s level of activity). Our results support Wolff’s law and show the highest variance in the axial direction and the lowest variance in the radial direction