Does the Novel Thulium Fiber Laser Have a Higher Risk of Urothelial Thermal Injury than the Conventional Holmium Laser in an In Vitro Study?

Introduction and Objective: The novel thulium fiber laser (TFL) has been shown to break stones more rapidly than the holmium:YAG laser (HL). However, some evidence suggests that the TFL generates more heat. The purpose of this study is to compare ureteral temperatures generated by these lasers during ureteroscopic laser lithotripsy in a benchtop model. Methods: A 1-cm BegoStone was manually impacted in the proximal ureter of a three-dimensional printed kidney-ureter model and submerged in 35.5°C saline. Lithotripsy was performed using a 7.6F flexible ureteroscope and a 200 μm laser fiber without a ureteral access sheath. The Dornier 30 W HL, Olympus 100 W HL, and Olympus 60 W TFL were compared. A needle thermocouple to measure temperature was inserted 2 mm from the laser tip. Irrigation was maintained at 35 cc/minute at room temperature using the Thermedx FluidSmart System. Intraluminal temperature was continuously recorded for 60 seconds of laser activation. Five trials were performed for each of four different power settings: 3.6, 10, 20, and 30 W. Analysis of variance and Mann-Whitney U tests were performed with p < 0.05 considered significant. Results: Intraureteral fluid temperature increased as laser power settings increased for all lasers (p < 0.05). The TFL generated higher average ureteral fluid temperatures than the Dornier and Empower HL at all power settings tested (p < 0.001). The maximum temperature for the TFL was higher than the Dornier and Empower HL at all power settings tested (p < 0.001), except at 20 W with the Empower HL. At 30 W, the TFL exceeded 43°C, the threshold for tissue damage. Conclusions: The TFL generated more heat at all settings tested. Supraphysiologic ureteral temperatures may be generated with extended use at high energy settings and low irrigation rates. Understanding the heat generation properties of both lasers could help improve the safety of ureteroscopic laser lithotripsy.