Differences in sensitivity to microstructure between cyclic- and impact-based microindentation of human cortical bone.

Unlike the known relationships between traditional mechanical properties and microstructural features of bone, the factors that influence the mechanical resistance of bone to cyclic reference point microindention (cRPI) and impact microindention (IMI) have yet to be identified. To determine whether cRPI and IMI properties depend on microstructure, we indented the tibia mid-shaft, the distal radius, and the proximal humerus from 10 elderly donors using the BioDent and OsteoProbe (neighboring sites). As the only output measure of IMI, bone material strength index (BMSi) was significantly different across all three anatomical sites being highest for the tibia mid-shaft and lowest for the proximal humerus. Total indentation distance (inverse of BMSi) was higher for the proximal humerus than for the tibia mid-shaft but was not different between other anatomical comparisons. As a possible explanation for the differences in BMSi, pore water, as determined by 1 H nuclear magnetic resonance, was lowest for the tibia and highest for the humerus. Moreover, the local intra-cortical porosity, as determined by micro-computed tomography, was negatively correlated with BMSi for both arm bones. BMSi was also positively correlated with peak bending stress of cortical bone extracted from the tibia mid-shaft. Microstructural correlations with cRPI properties were not significant for any of the bones. The one exception was that average energy dissipated during cRPI was negatively correlated with local tissue mineral density in the tibia mid-shaft. With higher indentation force and larger tip diameter than cRPI, only IMI appears to be sensitive to the underlying porosity of cortical bone.