Impact of microscale properties measured by 50-MHz acoustic microscopy on mesoscale elastic and ultimate mechanical cortical bone properties

Cortical bone strength is determined by multiple factors. Among them, changes in cortical porosity and elastic properties of the extracellular bone matrix have been suggested to be related to fracture risk. The objective of this ex-vivo study was to assess matrix stiffness and porous structure by means of 50-MHz acoustic microscopy in native samples obtained from the tibia mid-shaft of a representative sample cohort (19 human donors, age range: 69-94 yrs). These microscale properties were compared to elastic and ultimate mesoscale properties of cuboidal sections from the same samples, which were assessed by means of mechanical testing. After binarization and definition of the regions of interest (ROI) for mechanical testing, the mean stiffness, average pore diameter, porosity and number of large resorption lacunae (N.RL) were evaluated in each ROI. Thereafter, cuboidal blocks were cut from each sample at the ROI locations and forwarded to mechanical investigations. The cuboidal samples (n=48) were tested under moist conditions using a material testing machine. After preconditioning (three times, strain rate:.05 min^-1) testing was conducted until mechanical failure occurred. Young's modulus, ultimate compressive strength and yield point were extracted from the stress-strain curves. Matrix stiffness c₃₃ ranged from 22.4 GPa to 46.9 GPa (CV: 19%). Cortical porosity and pore density varied between 3.5% and 34% (CV: 52.7%), and 9.2 mm-2 and 37.8 mm-2, respectively. Young's Modulus and ultimate strength ranged from 8.6 GPa to 19.6 GPa (CV: 18.7%) and 117.1 MPa to 201.6 MPa (CV: 14.7%), respectively. Young's modulus and ultimate strength were significantly correlated (R=.72) and both properties were associated with N.RL (-.55 <R <-.52), but not with the matrix stiffness. These findings underline the diagnostic values of resorption lacunae and mesoscale Young's modulus for the prediction of bone strength. The latter could be assessed by in-vivo US technologies, e.g. axial transmission measurements.