Holmium: YAG (λ=2120 nm) vs. Thulium fiber laser (λ=1908 nm) ablation of kidney stones: Thresholds, rates, and retropulsion

The Holmium:YAG (Ho:YAG) laser lithotriptor is capable of operating at high pulse energies, but its efficient operation is limited to relatively low pulse rates (∼10 Hz) during lithotripsy. On the contrary, the Thulium Fiber Laser (TFL) is limited to low pulse energies, but can operate at very high pulse rates (up to 1000 Hz). This study compares stone ablation threshold, ablation rate, and retropulsion effects for different Ho:YAG and TFL operation modes. The TFL (λ=1908 nm) was operated with pulse energies of 5-35 mJ, 500-μs pulse duration, and pulse rates of 10-400 Hz. The Ho:YAG laser (λ=2120 nm) was operated with pulse energies of 30-550 mJ, 350-μs pulse duration, and pulse rate of 10 Hz. Laser energy was delivered through small-core (200-270-μm) optical fibers in contact mode with human calcium oxalate monohydrate (COM) stones for ablation studies and plaster-of-Paris stone phantoms for retropulsion studies. The COM stone ablation threshold for Ho:YAG and TFL measured 82.6 J/cm²and 20.8 J/cm², respectively. Stone retropulsion with Ho:YAG laser increased linearly with pulse energy. Retropulsion with TFL was minimal at pulse rates <150 Hz, then rapidly increased at higher pulse rates. For minimal stone retropulsion, Ho:YAG operation at pulse energies <175 mJ at 10 Hz, and TFL operation at 35 mJ at 100 Hz is recommended, with both lasers producing comparable ablation rates. Further development of a TFL operating with both high pulse energies (e.g. 100-200 mJ) and high pulse rates (100-150 Hz) may also provide higher ablation rates, when retropulsion is not the primary concern.