On the cortical bone elasticity, toughness and bone quality
Résumé
This study aims at investigating the relationship between different aspects of bone quality and understanding how bone quality may affect stiffness at the mm-scale and toughness. The transverse isotropic effective stiffness tensor was measured by resonant ultrasound spectroscopy (RUS) and the vascular porosity was estimated from synchrotron radiation micro-computed tomography (SR-¼CT) images. A cross-section at the mid-diaphysis was sectioned from the left femur of 27 human cadavers. Then, a set of 54 parallelepiped-shape samples (size 3x4x5 mm3) was prepared for RUS measurements and SR-¼CT imaging at ERSF (voxel size 6.5 µm). Another set of adjacent 54 samples (size 3x4x25 mm3) was harvested for mechanical testing and the remaining material was used for cross-links and collagen quantification. These last samples were divided into two samples (1x2x25 mm3) then notched (1 mm length) in their middle, in the transverse direction. Three-point bending tests were conducted at two strain rates 10-4 s-1 and 10-1 s-1 to measure toughness (quasi-static KJcStat and dynamic KJcDyn). Specimen dimensions are defined by the anatomical shape of femoral diaphysis: radial (axis 1), circumferential (axis 2) and axial direction (axis 3). Immature and mature collagen crosslinks were separated by reversed-phase HPLC method and quantified using fluorescence and mass spectrometry detection. Relationships between variables were investigated with Spearman's rank correlation coefficients. The bone-quality markers analysed here seem to differently affect toughness and stiffness. Indeed, markers that showed an effect in toughness did not in stiffness. Vascular porosity was significantly correlated with all stiffness coefficients but not with toughness. Enzymatic immature cross-links significantly correlated with stiffness coefficients, while only the non-enzymatic cross link pentosidine significantly affected toughness. Finally, the existing correlations between elastic parameters and toughness are shown in Table 1. Knowing the effects of these markers in the mechanical properties of bone may provide a more precise estimate for the fracture risk.