Documenting the Anisotropic Stiffness of Hard Tissues with Resonant Ultrasound Spectroscopy.

Recent advances in resonant ultrasound spectroscopy (RUS) leverage accurate measurements of the anisotropic stiffness of hard tissues at millimeter scale. RUS is the only available technique to date to assess the entire stiffness tensor of bone from a unique rectangular parallelepiped specimen. Accurately measured stiffness constants are required for bone mechanics models and may provide information on some fundamental aspects of hard tissues biology such as regulation of bone mass, remodeling and healing. In this chapter, we review the anisotropic stiffness data of human hard tissues measured with RUS, mostly during the last decade. Hard tissues covered here include human enamel and dentin, cortical bone from the femur and tibia of human adults, and child cortical bone tissue, accounting for 288 specimens in total. Data was collected in the literature and from previous works of our group. We performed a comparative study to depict the differences in the elastic properties of these hard tissues. Our objectives were to: (1) document the range of anisotropic stiffness constants in human hard tissues (orthotropic or transverse isotropic symmetry); and (2) provide empirical laws between mass density and anisotropic stiffness of cortical bone at different skeletal sites. Finally, we discuss the challenges and perspectives to use RUS for large collections of specimens.