Safety of a Novel Thulium Fiber Laser for Lithotripsy: An In Vitro Study on the Thermal Effect and Its Impact Factor

Introduction: To investigate the thermal effect on the water by a novel thulium fiber laser (TFL) designed for lithotripsy and evaluate the safety of this laser for clinical use. Materials and Methods: An in vitro experimental setup was constructed. A test tube filled with saline was immersed in an electric water bath, and a TFL fiber and a thermal probe were inserted into it. Saline was irrigated into the tube and pumped out synchronously at the same speed by two pumps, respectively, to maintain convection when needed. Then, continuous TFL firing of different power settings was imposed to saline in the tube for 60 seconds, on the conditions of different irrigation rates. The temperature was recorded every 5 seconds during the whole trial, and each trial was repeated five times. Safety threshold of temperature increase (STTI) was determined comparing with the deemed safe temperature of 43°C in vivo. Results: On condition of 0 mL/min irrigation rate, STTI was 6.5°C, and water temperature increase (WTI) caused by ≥15 W settings surpassed STTI after 20 seconds of laser firing; on condition of 15 mL/min irrigation rate, only WTI caused by the highest 30 W power setting surpassed STTI after 45 seconds of laser firing. When irrigation rate was added up to 25 and 50 mL/min, WTIs caused by all power settings were below STTIs in a 60-second experiment. High frequency and low pulse energy combinations caused a slightly higher WTI compared with low frequency and high pulse energy, given a constant power and irrigation rate. Conclusion: Power setting and irrigation rate collaboratively play a critical role in WTI during TFL lithotripsy, and it is safe to use TFL referring to the thermal effect as long as there is moderate irrigation, while TFL power should be lowered enough when irrigation is ceased.