Histotripsy in collagenous tendons.

Tendon injuries comprise 45% of the 66 million musculoskeletal injuries each year with annual costs of $114 billion. Conservative therapies to treat injured tendon produce mixed success rates. Histotripsy is a high intensity focused ultrasound modality that creates and oscillates bubbles to fractionate soft tissues in a well-defined focal volume; however, highly collagenous tissues like tendon have proven resistant to histotripsy. The goal here is to explore acoustic parameters for mechanical fractionation of tendons. Large animal Achilles tendons were exposed to HIFU at 1.07-3.68 MHz with 0.005-20 ms pulses repeated at 1-1000 Hz for 15-60 s with acoustic pressures up to p+ = 97 MPa, p- = 27 MPa. A Verasonics research ultrasound imaging system and ATL L7-4 transducer was used to monitor bubble activity during exposure. Tendon samples were fixated and stained with Hematoxylin and Eosin (H&E) for cellular morphology analysis and alpha-nicotinamide dinucleotide diaphorase (αNADH-d) for enzymatic activity analysis. Results showed successful bubble creation in the focal region; however, only 1/90 samples showed focal coagulative necrosis histologically. Future work involves an evaluation of whether bubbles are of insufficient size or collapse strength to mechanically fractionate tendon. [Work supported by Penn State Seed Grant, NIH EB027886, and NSF DGE1255832].