Breaking urinary stones to small size with burst wave lithotripsy

It is hypothesized a 2.6-mm stone was too small to break with 390-kHz burst wave lithotripsy (BWL). In clinical trial NCT03873259, a 2.6-mm stone failed to break after 10 min of 390-kHz BWL and was removed intact. Ex vivo, the stone failed to break after another 30 min at 390 kHz but broke into four pieces in 4 min at 650 kHz. A linear elastic model was used to calculate the stress created inside stones of different sizes, shapes, and compositions by shock wave lithotripsy (SWL) and different BWL frequencies. The model predicts above a threshold frequency proportionate to wave speed over stone length, the maximum principal stress inside a stone increases to more than five times the acoustic pressure applied. Thus, smaller stones may fragment at higher but not lower frequency. Amplification remains with irregularly shaped stones but is not seen with an SWL waveform. Ex vivo, stones smaller than 3 mm broke fastest at 830 kHz while larger stones broke fastest with 390 kHz followed by 830 kHz. For small stones and fragments, increasing BWL frequency many produce, amplified stress in the stone causing the stone to break to smaller fragments to pass. [Work supported by NIH-P01-DK04331 and NIH-K01-DK104854.]