Nonlinear resonant ultrasound spectroscopy (NRUS) for bone micro-damage assessment

Abstract

Bone micro-damage has a strong influence on bone quality and fracture risk. It is therefore important to develop a non invasive tool for bone micro-damage assessment. Nonlinear Resonant Ultrasound Spectroscopy (NRUS) is a technique exploiting the significant nonlinear behaviour of damaged materials, related to the presence of damage. This study shows for the first time the feasibility of this technique for damage assessment in bone. Two samples of bovine cortical bone were subjected to a progressive damage experiment. After removing soft tissues and marrow, the samples were wrapped in gauze impregnated with saline solution in order to keep them hydrated during the experiment. Damage accumulation was progressively induced in the samples by compressional fatigue cycling in a mechanical testing press. At each damage step, NRUS experiments were performed on the samples in order to study their resonance behavior. Each sample was probed using a step-sweep in frequency around an eigenmode of the sample, at gradually increasing drive levels. The measured resonance frequency shift provided quantitative information regarding the nonlinear behavior of the sample. For independent assessment of damage, high energy X-ray CT imaging was performed, but only helped in the detection of the prominent cracks. As the quantity of damage accumulation increased, NRUS revealed a corresponding increase in the nonlinear response. The measured change in nonlinear response is much more sensitive than the change in modulus. Indeed, the measured nonlinear parameter, proportional to the frequency shift, revealed a 700% increase and a 1400% increase in the two samples respectively, whereas no quantitative change in the modulus could be measured. The results suggest the inability of linear acoustic parameters to reflect mechanically induced damages and that NRUS could be a potential tool for micro-damage assessment in bone. Further work has to be carried out for a better understanding of the physical nature of damaged bone, and for the ultimate goal of in vivo implementation of the technique.