Defining the safe operational window for holmium laser lithotripsy in impacted ureteral stones: an analysis of power, operator duty cycle, and irrigation flow

Iatrogenic thermal injury is a proposed mechanism for ureteral strictures that may occur following holmium laser lithotripsy for impacted stones. The safe operational parameters, including laser power, operator duty cycle (ODC), and irrigation flow rate, remain poorly defined. We utilized a high-fidelity, ex vivo 3D-printed silicone model to simulate the scenario of an impacted ureteral calculus. A full-factorial experimental design was employed to evaluate all combinations of laser power (10, 20, 30, and 40 W), irrigation flow rates (10 and 20 mL/min), and 50% ODC patterns (1 s/1 s, 3 s/3 s, and 5 s/5 s). The primary outcome was the cumulative thermal dose (CEM43) accumulated over a 90-s operational period. At power levels of 10 W and 20 W, nearly all combinations resulted in negligible CEM43 accumulation, measured at less than 0.06 min. A critical transition occurred at 30 W, where the combination of low flow (10 mL/min) and a prolonged laser-on time (5 s) resulted in CEM43 levels exceeding 120 min within the 90-s experiment, specifically reaching 336.18 min. At 40 W, the thermal risk was substantially elevated. Increasing the irrigation flow from 10 to 20 mL/min at 40 W, while maintaining a 3-s laser-on time, reduced CEM43 levels from a hazardous 309.80 min to 11.11 min. However, a 5-s laser-on time at 40 W consistently resulted in hazardous CEM43 levels, regardless of the flow rate. Thermal risk during laser lithotripsy is governed by the interplay of power, ODC, and flow. In this ex vivo model, 10 W was safe across all tested conditions. While 20 W is generally safe, prolonged activation at low flow should be avoided. Risk increases markedly above 30 W, where safety requires high flow and short activation bursts, conditions with a safety margin in theory, but not reproducibly so in practice. Moreover, because CEM43 is cumulative, the 90-s experimental window does not reflect total clinical activation time; longer activation may exceed injury thresholds even for settings that appeared safe in this model. Clinicians should therefore interpret these data with caution and use lower power setting.