Redefining the Role of Vapor Bubble Dynamics in Laser Lithotripsy

Abstract

Urinary stone disease (USD) is the second most costly urologic condition in the US, with a healthcare expenditure over $2 billion/year. Laser lithotripsy (LL) with Holmium:YAG laser (λ = 2.12 µm) has been the primary treatment of choice for USD since 2000. However, recent introduction of high power/high frequency lasers, especially the thulium fiber laser (λ = 1.94 µm), have offered urologists more treatment options (e.g., dusting vs. fragmenting vs. pop-dusting) yet also raised serious concerns regarding the lack of physical understanding about the dissimilar characteristics of various laser technologies. This knowledge gap significantly hinders efforts to determine the optimal laser settings and treatment strategies to improve patient outcomes while minimizing potential thermal injuries.The overarching goal of this dissertation is therefore to comprehensively investigate the mechanisms of stone damage, temperature rise and associated thermal tissue injury risk. Specifically, the role of vapor bubble has been re-defined in three critical processes during LL: 1) the collapse of toroidal bubble with progressively intensified shock wave emission has been demonstrated as the primary mechanism for stone dusting via both in vitro experiments and a multiphysics model, 2) the intra-crater bubble expansion has been identified as a new mechanism for stone fracture; both findings are distinctly different from the prevailing theory of photothermal ablation for stone damage in LL in the past two decades, and 3) convective heat transfer with rigorous mixing induced by bubble pulsation and irrigation flow has been shown to be critical in assessing the maximum fluid temperature inside the kidney, and both experiments and modeling support a simple relation correlating the temperature rise with laser power, irrigation fluid temperature and flow rate. Overall, this dissertation creates paradigm-changing knowledge regarding the mechanisms of action in LL, and furthermore, provides model-based guidelines that will help urologists to judiciously select the laser and irrigation flow settings for safe and effective treatment of USD patients.