Axial transmission measurements of guided modes on bone phantoms and in vitro human bone specimen

Abstract

Cortical bone porosity has been evidenced as being a major if not the major 'footprint' of bone loss and fragility. Several studies report that cortical bone behaves like a waveguide. Measurements of guided mode wavenumbers together with appropriate waveguide modelling have therefore the potential for providing estimations of effective stiffness coefficients (which are largely determined by cortical porosity) and also cortical thickness. However, data interpretation is challenging due to the heterogeneous, dissipative and irregular nature of the wave guide. Moreover surrounding and internal tissues modify the guided modes. This paper presents measurement of the wavenumbers of guided wave modes, using a multi-transmitter multi-receiver axial transmission probe. The method has been previously validated on isotropic elastic and visco-elastic plates covered by a soft tissue-mimicking layer. As an extension of our previous works, we have conducted in vitro experiments on (i) bone-mimicking tubes without and with soft-tissue mimickning layers and on (ii) human radius and tibia specimens. Despite challenging measurement conditions owing to low signal-to-noise ratio, guided mode wavenumbers have been successfully measured. The results were consistent with a free plate model. Data inversion led to an reasonably accurate estimate of the shell thickness and bulk velocities for all cases.