Comparison of 1908 and 1940 nm wavelengths for thulium fiber laser lithotripsy

Abstract

The experimental Thulium fiber laser (TFL) is currently being explored as an alternative to the gold standard Holmium:YAG laser for lithotripsy. The TFL emits laser radiation at two primary wavelengths, 1908 or 1940 nm, which closely match high and low temperature water absorption peaks in tissue, respectively. Water is a primary absorber of infrared laser radiation, and is present in the pores of kidney stones in the urinary tract. Constant saline irrigation is also applied through the ureteroscope working channel to clear stone debris and improve visibility and safety during lithotripsy. Previous studies have shown that the water absorption peak shifts from 1940 nm to 1920 nm, as water temperature increases heating. At high water temperatures, the absorption coefficient (μa) 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 Thulium fiber lasers (1908 and 1940 nm) were tested at similar laser parameters of 35 mJ energy per pulse, 500 μs pulse duration, 300 Hz pulse rate, and 10.5 W average power, with energy delivered through 200-μm-core optical fibers. The handheld fiber was maintained in contact with 6-9 mm diameter uric acid (UA) stones, immersed in a saline bath with saline flow (n=10 stones per group). Time to fragment and pass all stone fragments through a 1 mm sieve was measured, and then divided into initial stone mass to calculate ablation rate. For each laser group, 1908 and 1940 nm, initial stone mass was 270 ± 60 mg vs. 260 ± 50 mg, respectively (p = 0.9). Stone ablation rates measured 0.9 ± 0.2 and 0.9 ± 0.1 mg/s, respectively (p = 0.9). Stone ablation thresholds also measured 8 ± 7 and 5 ± 13 J/cm2, respectively (p = 0.8). There was no significant difference in UA stone ablation thresholds and ablation rates between 1908 and 1940 nm TFL wavelengths.