Microstructural and compositional contributions towards the mechanical behavior of aging human bone measured by cyclic and impact reference point indentation

Abstract

The assessment of fracture risk often relies primarily on measuring bone mineral density, thereby accounting for only a single pathology: the loss of bone mass. However, bone's ability to resist fracture is a result of its biphasic composition and hierarchical structure that imbue it with high strength and toughness. Reference point indentation (RPI) testing is designed to directly probe bone mechanical behavior at the microscale in situ, although it remains unclear which aspects of bone composition and structure influence the results at this scale. Therefore, our goal in this study was to investigate factors that contribute to bone mechanical behavior measured by cyclic reference point indentation, impact reference point indentation, and three-point bending. Twenty-eight female cadavers (ages 57-97) were subjected to cyclic and impact RPI in parallel at the unmodified tibia mid-diaphysis. After RPI, the middiaphyseal tibiae were removed, scanned using micro-CT to obtain cortical porosity (Ct.Po.) and tissue mineral density (TMD), then tested using three-point bending, and lastly assayed for the accumulation of advanced glycation end-products (AGEs). Both the indentation distance increase from cyclic RPI (IDI) and bone material strength index from impact RPI (BMSi) were significantly correlated with TMD (r=-0.390, p=0.006; r=0.430, p=0.002; respectively). Accumulation of AGEs was significantly correlated with IDI (r=0.281, p=0.046), creep indentation distance (CID, r=0.396, p=0.004), and BMSi (r=-0.613, p<0.001). There were no significant relationships between tissue TMD or AGEs accumulation with the quasi-static material properties. Toughness decreased with increasing tissue Ct.Po. (r=-0.621, p<0.001). Other three-point bending measures also correlated with tissue Ct.Po. including the bending modulus (r=-0.50, p<0.001) and ultimate stress (r=-0.56, p<0.001). The effects of Ct.Po. on indentation were less pronounced with IDI (r=0.290, p=0.043) and BMSi (r=-0.299, p=0.037) correlated modestly with tissue Ct.Po. These results suggest that RPI may be sensitive to bone quality changes relating to collagen.

Keywords: Advanced glycation end-products; Aging; Bone mechanics; Bone viscoelasticity; Damage mechanics; Reference point indentation.

Figure 1

Study design. Material properties were obtained at three different loading rates for a mid-diaphysis section of the anterior tibia using three-point bending, cyclic reference point indentation, and impact reference point indentation. Compositional data including cortical porosity, tissue mineral density, and accumulation of advanced glycation end-products were also acquired.

Figure 2

Relationship between low- and high-strain rate indentation testing techniques. The impact indentation produced Bone Material Strength index (BMSi) was significantly correlated with A) indentation distance increase, B) average creep indentation distance, C) average energy dissipated, D) loading slope, and E) unloading slope. Panels A–E show Spearman’s correlations. F) Bland-Altman plot of the total indentation distance for both devices. The scatter between devices increases with decreasing bone quality as indicated by an increased average.

Figure 3

Tissue mineral density relationships. A) TMD was not significantly correlated with toughness. B&C) Both indentation techniques were significantly correlated with TMD.

Figure 4

Accumulation of advanced glycation end-products relationships. A) Modulus of toughness was not significantly correlated with AGEs content. B) IDI from cyclic reference point indentation testing was only moderately correlated with AGEs. However, C) BMSi from impact reference point indentation was most strongly correlated with AGEs.

Figure 5

Cortical porosity relationships. All mechanical testing were correlated with Ct.Po. with the A) modulus of toughness being the strongest. B, C) Indentation testing techniques were only modestly correlated.