Bone Abstracts

For a reliable assessment of bone’s material quality in a clinical environment, a fast way to measure the mechanical properties of bone is needed. The investigation of material heterogeneity and anisotropy resulting from bone remodeling and mineralization requires an imaging technique with micrometer resolution. Scanning acoustic microscopy (SAM) using high-frequency lenses allows measuring the stiffness of bone under wet conditions in a non-destructive way with this spatial resolution. The detected reflectivity of the acoustic waves includes information of both the stiffness and the mass density.

Several regions of human cortical bone of a femur were investigated, which included newly formed osteons and interstitial bone. For all the regions three measurements were performed: quantitative backscattered-electron imaging (qBEI) and SAM measurements using lenses with two different frequencies (400 and 820 MHz). The information about the mass density from qBEI, allows a calculation of an effective stiffness from the combined information of the two SAM images. An important technical aspect in the evaluation of the acoustic signal was the consideration of the correct opening angle of the acoustic lens.

Compared to the mass density of bone in osteons (1910 kg/m³), the higher mineral content in interstitial bone results in a 9% increase of the density. The contrast in stiffness, however, is more drastic. Compared to an average effective stiffness of 15 GPa in osteons, in interstitial bone this quantity is more than 25% increased. Beside this mechanical contrast on the level of the BSU, SAM maps show oscillations in the effective stiffness with a wavelength of the typical bone lamella thickness of ~5 μm in both osteons and interstitial bone. Also the amplitude of these oscillations of about 3 GPa is similar in both regions. This mechanical heterogeneity can be explained by the anisotropic arrangement of the mineralized tissue.