Anisotropy of wave velocity in bone matrices of young and aged mice

The bone matrix of loaded bones, such as the femur and tibia, shows a uniaxial character along the loading direction. In contrast, non-loaded bones, such as the skull, are known to have more complex two-dimensional orientations due to the alignment of hydroxyapatite and collagen. Understanding this anisotropy is important for optimizing ultrasound therapy. To minimize the influence of bone microstructure, this study used the micro-Brillouin scattering (μ-BS) technique, which provides high spatial resolution (∼10 μm) for evaluating the bone matrix. A total of six bone samples were used: two each from aged male and female mice (approximately 100 weeks old) and two from young adult mice (30 weeks old). In the tibia, the maximum wave velocity was observed near the loading direction (within ±30°), indicating almost uniaxial anisotropy. In contrast, in the skull, the direction of maximum velocity varied among samples. In one aged female sample, the direction of maximum wave velocity was perpendicular to the anterior–posterior axis. The velocity ratio (maximum/minimum) was 97.1 ± 0.2% in the tibia and 96.5 ± 0.9% in the skull. No significant differences in the degree of anisotropy were found between young and aged mice, or between tibial and skull bones.