IEEE Trans Ultrason Ferroelectr Freq Control. 2021 Apr 23;PP. doi: 10.1109/TUFFC.2021.3075375. Online ahead of print.
Thirty million tendon injuries occur annually in the US costing $114 billion. Conservative therapies, like dry needling, promote healing in chronically injured tendon by inducing microdamage but have mixed success rates. Focused ultrasound (fUS) therapy can noninvasively fractionate tissues through the creation, oscillation, and collapse of bubbles in a process termed histotripsy; however, highly collagenous tissues, like tendon, have shown resistance to mechanical fractionation. This study histologically evaluates whether fUS mechanical disruption is achievable in tendon. Ex vivo rat tendons (45 Achilles and 44 supraspinatus) were exposed to 1.5 MHz fUS operating with 0.1-10 ms pulses repeated at 1-100 Hz for 15-60 s with peak positive pressures <89 MPa and peak negative pressures <26 MPa; other tendons were exposed to dry needling or sham. Immediately after treatment, tendons were frozen fixed and stained with Hematoxylin and Eosin (H&E) or alpha-nicotinamide adenine dinucleotide diaphorase (α-NADH-d) and evaluated by two reviewers blinded to the exposure conditions. Results showed successful creation of bubbles for all fUS-treated samples; however, not all samples showed histological injury. When injury was detected, parameter sets with shorter pulses (0.1-1 ms), lower acoustic pressures, or reduced treatment times showed mechanical disruption in the form of fiber separation and fraying with little to no thermal injury. Longer pulses or treatment times showed a combination of mechanical and thermal injury. These findings suggest mechanical disruption is achievable in tendon within a small window of acoustic parameters, supporting the potential of fUS therapy in tendon treatment.