Intra-Crater Bubble Expansion Drives the Fracture of Impacted Ureteral Stones in Laser Lithotripsy

To investigate the fracture mechanism of impacted ureteral stones during laser lithotripsy (LL). Impacted 6 x 6 mm cylindrical BegoStone samples embedded in a hydrogel ureter model were treated using either Holmium:YAG (Ho:YAG) laser or Thulium Fiber Laser (TFL) via three clinical strategies: "drill and core", contact, and non-contact modes. Laser pulses were delivered using three pulse energy/frequency settings: 0.8 J/12 Hz, 1.0 J/10 Hz, and 1.2 J/8 Hz with a 3 s on/3 s off protocol, under continuous irrigation at 40 mL/min. Crater formation, surface crack development, and bubble dynamics were assessed via optical coherence tomography, video, and high-speed photography. To delineate the contributions of different plausible damage mechanisms, bubble collapse was suppressed by leveraging the ureteroscope's proximity effect, and thermal ablation was minimized by treating donut-shaped stones with a central tunnel. The role of bubble expansion in stone fracture was further evaluated by systematically varying tunnel size or pulse energy. Surface cracks and stone fracture were observed exclusively in Ho:YAG laser-treated stones using the "drill and core", but not the other two strategies. TFL produced deeper craters yet failed to induce any significant crack formation or propagation under all conditions. Suppression of bubble collapse or thermal ablation had minimal effect on surface crack formation and growth. In contrast, the extent and number of surface cracks correlated strongly with the maximum lateral diameter and expansion rate of the vapor bubbles - two parameters that were significantly greater for Ho:YAG laser than TFL. Importantly, the crack formation also showed an inverse correlation with the size of the initial crater or tunnel in the stone. Intra-crater bubble expansion, rather than thermal ablation or bubble collapse, is the primary mechanism driving the fracture of impacted ureteral stone in LL.