In a number of conditions, both natural (e.g. aging, osteoporosis) and artificial (e.g. radiation therapy, space flight), bone mechanical properties has been shown to vary. Bone strength is determined by the density of bone as well as its quality. While bone mineral density can be measured in a clinical setup, bone quality is affected by a variety of factors such as trabecular connectivity and cortical porosity, crystalline composition and the organic matrix, and by the integrity of the material, for example presence of micro-cracks. Little is known about these properties of bone, partially because techniques for measuring specific material and structural properties of bone in vivo are not commonly available. While bone strength can be only measured directly by mechanical testing, it has been shown that the modulus of elasticity can be indicative of the ultimate bone strength. Thus, non-invasive approaches to assess bone strength by measuring ultrasound propagation velocity have been proposed. Currently available clinical methods are based on transmission ultrasound, thus subject to limitations in measurement location and to detrimental effects due to overlying soft tissue and bone marrow content. Ultrasound Critical-angle Reflectometry (UCR) is capable to measure both the bone elasticity and its anisotropy using the reflected ultrasound amplitude, thus overcoming these shortcomings.
Schematic of the UCR principle and bone elastic modulus.
The method allows measurement of cortical bone elastic coefficient in vivo and it is currently used for research in the treatment of osteoporosis as a complement to BMD.
Cortical elasticity in clinical osteoporotic patients.
Equally challenging areas of research in which UCR can be applied are the assessment of changes of bone properties due to radiation therapy, and the bone loss in the absence of gravity and the search for appropriate countermeasures. The development and the experimental validation of the theoretical model of UCR for converging ultrasound waves, and the interaction of such waves with the structurally complex bone composed of both cortical and trabecular material is one of the future directions of this research.
UCR detector using a focused ultrasound wave.