Ejecta-Modulated Bubble Dynamics Play a Dominant Role in Stone Retropulsion.

Cavitation bubbles generated by laser absorption in liquids collapse violently, producing high-speed jets, toroidal bubbles, and shock waves that induce material erosion and object displacement. In laser lithotripsy, this phenomenon causes kidney stone migration (retropulsion) which reduces procedural efficiency and requires frequent repositioning of the fiber tip. Retropulsion has traditionally been attributed to recoil momentum from laser-generated ejecta; however, our experiments demonstrate that vapor bubble dynamics, rather than ejecta recoil, predominantly govern stone motion, while ejecta modulate bubble morphology and collapse asymmetry. A clinical Ho:YAG laser system was used to deliver pulses at varying stand-off distances to freely suspended Begostone phantoms. Ultra-high-speed imaging up to 5 million frames per second, complemented by optical coherence tomography, captured the coupled evolution of bubble lifecycles, ejecta behavior, crater formation, and stone displacement in both air and water. Analysis reveals that while ejecta content decreases with continued ablation, bubble-driven forces persist and dominate retropulsion. Asymmetric crater geometries formed by earlier pulses influence subsequent bubble shape and collapse, altering both the magnitude and direction of stone movement. A dimensionally consistent empirical model based on geometric, temporal, and morphological parameters quantitatively describes these effects. The results demonstrate that ejecta-modulated bubble dynamics-wherein ejecta shape the crater geometry and induce bubble asymmetry that guides collapse-jet formation and the resulting bulk fluid momentum transfer-constitute the primary physical mechanism underlying stone retropulsion, providing new insights for optimizing laser delivery strategies in lithotripsy and for controlled cavitation applications in medicine and industry.