Comparison of first-generation (1908 nm) and second-generation (1940 nm) thulium fiber lasers for ablation of kidney stones

Thulium fiber lasers (TFL) are being developed as alternatives to conventional holmium:YAG lasers for lithotripsy. TFLs emit at two primary infrared (IR) wavelengths, 1908 or 1940 nm, closely matching high (70 degrees C) and low (22 degrees C) temperature water absorption peaks in tissue, respectively. Water, which is present in the pores of kidney stones and fluid environment of the urinary tract as well as delivered through the working channel of the ureteroscope during lithotripsy, is a primary absorber of IR laser radiation. The water absorption peak shifts from 1940 to 1920 nm with increasing water temperature during laser ablation. At high water temperatures (70 degrees C), the absorption coefficient is 150 cm(-1) at 1908 nm and 135 cm(-1) at 1940 nm. The goal of this study was to determine whether this 10% difference translates into a measurable difference in kidney stone ablation rates. Two TFLs (1908 and 1940 nm) were tested at similar laser parameters of 35 mJ energy/pulse, 500-mu s pulse duration, 300-Hz pulse rate, and 10.5-W average power, using 200-mu m-core silica optical fibers. The handheld fiber was maintained in contact with 6- to 9-mm diameter uric acid (UA) stones, immersed in a saline bath with saline flow (n = 10 stones / group). Laser irradiation time to fragment and pass all stone fragments through a 1-mm sieve was measured and then divided into initial stone mass to calculate stone ablation rates. For each laser group (1908 and 1940 nm), initial stone mass was 270 +/- 60 mg versus 260 +/- 50 mg, respectively (p = 0.9). Stone ablation rates measured 0.9 +/- 0.2 and 0.9 +/- 0.1 mg s(-1) (p = 0.9). Stone ablation thresholds measured 8 +/- 7 and 5 +/- 13 J cm(-2) (p = 0.8). There was no significant difference in UA stone ablation thresholds and ablation rates between 1908- and 1940-nm wavelengths. Newer, more compact, efficient, and higher peak power TFLs operating at 1940 nm provide comparable stone ablation rates to older 1908-nm TFLs for similar laser parameters. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)