Ultrasonic damage assessment of articular cartilage for ultrasonic drug delivery

Osteoarthritis (OA) is a common musculoskeletal disease. There are drug therapies under development, but no localized way to transfer drugs into articular cartilage (AC). Previously we showed that 20 kHz high intensity ultrasound (HIU) can deliver micro- and nano-particles into AC [1]. However, at low frequencies, cavitation may occur. Our hypothesis is that 20 MHz ultrasound pulse-echo can be used to track changes in ultrasound reflection from AC during HIU. Here we tested the feasibility of this approach for characterizing cavitation-related damage. We observed that the relative decrease in mean surface reflection coefficient at 15, 20 and 25 MHz were -40.9 +/- 7.9 %, -53.4 +/- 12.0 % and -61.1 +/- 7.1 %, respectively, during a 5-minute HIU exposure. Frequencies above 16 MHz were the most sensitive to changes induced by HIU in both samples. We propose that the change resulted from cavitation-related cratering of the AC surface and exposure of AC tissue with lower collagen content. In conclusion, the acoustic reflection coefficient of the AC surface could be a sensitive parameter to estimate damage in the AC surface during HIU exposure.