Different Types of Bone Fractures Observed by Terahertz Time-Domain Spectroscopy

Microcracks are common in compact bone, but their continued propagation can lead to macroscopic fractures. These microcracks cannot be visualized radiographically, necessitating alternative noninvasive methods to identify excessive microcracking and prevent fractures. In this study, terahertz time-domain spectroscopy (THz-TDS) was used to examine bone interiors near cracks resulting from loading in bovine tibia samples. Various loading configurations, such as impact, quasi-static loading, and fatigue loading, known to induce different types of micro-scale damage, were applied. The values of refractive index and absorption coefficient of the bone samples were then determined from the THz-TDS spectra acquired before loading and after fracture. The study revealed that different loading configurations led to varying terahertz optical coefficients associated with various types of bone fractures. Specifically, the refractive index notably increased under fatigue loading but remained relatively stable during quasi-static bending. The absorption coefficient of bone decreased only under fatigue loading. Furthermore, samples were subjected to axial and radial impacts without sustaining damage. Results indicated that in the undamaged state, the change in refractive index was smaller compared to after impact failure, while the change in absorption coefficient remained consistent after failure. Under radial impact loading, changes in refractive index and absorption coefficient were significantly more pronounced than under axial loading. Prior to loading, the measured value of refractive index was 2.72 ± 0.11, and the absorption coefficient was 6.33 ± 0.09 mm⁻¹ at 0.5 THz.