The Role of Ureteroscope in Controlling Vapor Bubble Dynamics and Retropulsion during Holmium: YAG Laser Lithotripsy*

This study investigates the role of ureteroscope (URS) offset distances (OSD) from the fiber tip on vapor bubble dynamics and stone retropulsion during Holmium (Ho): YAG laser lithotripsy (LL). Irradiation of pulsed Ho: YAG laser (λ = 2100 nm) in water creates an elongated vapor bubble tunnel at the fiber tip, enhancing energy delivery to the stone surface. During collapse, the bubble generates forces that displace the stone away from fiber tip (i.e., retropulsion), thereby reducing ablation efficiency while prolonging procedure time due to frequent fiber realignment. Experiments with varying stand-off-distance (SD) and OSD, using high-speed imaging and optical coherence tomography, revealed that the URS modulates the Bjerknes attraction force between bubble and stone, and between bubble and URS, resulting in reduced stone displacement and increased crater volume formation. At OSD = 3 mm, attraction forces in short pulse (FWHM<200 µs) nearly eliminated retropulsion within the 0.6–1.2 J energy range for SD = 1 mm. Notably, the transition from attraction to retropulsion occurs at SD = 0.5–0.75 mm for short pulses and at OSD < 3 mm for long pulses (>200 µs). Both experiments and simulations demonstrated that the URS's presence led to more controlled bubble collapse and reduced retropulsion velocities. This approach shows promise in potentially increasing the LL efficiency through improved ureteroscope design.