Synergistic interaction between stress waves and cavitation is important for successful comminution of residual stone fragments in shock wave lithotripsy

Abstract

To assess the role of stress waves and cavitation in comminuting residual fragments during shock wave lithotripsy (SWL), cylindrical 4 × 4 mm BegoStone phantoms were treated in an electromagnetic lithotripter either at the focus (z = 0, p+ = ∼45 MPa) or pre-focally (z = −30 mm, p+ = ∼24 MPa). The treatment was performed with the stone immersed either in degassed water or in Butanediol, which has similar acoustic impedance to water but much higher viscosity to suppress cavitation. At the focus, the first fracture was observed after 26 ± 9 shocks, both in water and Butanediol (p = 0.7). However, when stones were moved pre-focally where comparable cavitation is produced (based on high speed imaging), the average shock number required for the initial fracture was increased to 66 ± 10 in water and 122 ± 20 in Butanediol (p = 0.002). Below −40 mm prefocally (p+ < 20 MPa), stones did not fracture in water even after 2,000 shocks, although cavitation was observed. Furthermore, stone comminution at the focus after 250 shocks was ∼35% in water compared to ∼5% in Butanediol (p < 0.001). Altogether, these findings suggest that a synergistic interaction between stress waves and cavitation is critical in producing effective stone comminution during SWL. [Work supported by NIH and NSF GRFP.]