1Human Evolutionary Biology, Harvard University, 2Human Evolution, Max Planck Institute for Evolutionary Anthropology
Saturday Morning, Alexander's
Analyses of locomotor function often assume that trabecular bone modifies to line up its principle material axes with the trajectory of the principal loads within a joint. Wolff’s so-called “law” of trabecular orientation, however, has not been tested experimentally in large mammals, and it is unclear how precisely or accurately one can predict peak load orientation in a joint from trabecular orientation. This question, moreover, cannot be answered using the comparative method, and requires controlled experimental testing. We tested the model using a sample of 20 sheep divided into three groups in which we varied tarsal but not carpal joint angles in two ways: (a) having some sheep run on inclined versus flat treadmills, and (b) having some sheep wear platform shoes on their forelimbs. Force plate analyses and kinematics were used to calculate an expected 3.6° difference in tarsal trabecular orientation, which was measured along with other parameters of trabecular bone architecture using microCT. As predicted, animals with more extended hindlimbs had 2.7- 4.3° (p<0.05) less obliquely oriented trabeculae in the bones of the tarsal but not the carpal joint. In addition, sheep that were exercised had a significant increase in trabecular bone volume (BV/TV) and trabecular number (Tb.N) and a significant decrease in trabecular spacing (Tb.Sp). These results demonstrate for the first time in large mammals that trabecular bone responds in vivo to a change in loading direction by altering its principle material axes and realigning with the trajectory of the new principal loads within a joint.
This study was funded by Harvard University, the Max Planck Institute and the Minerva Foundation.