Evaluation of Retropulsion Caused by Holmium: YAG Laser with Various Power Settings and Fibers

Abstract

The ideal intracorporeal lithotriper would comminute all types of calculi into small readily excreted particles. It would be small and flexible with an energy source safe for the uroepithelium. It should not break, should be inexpensive, and should not retropulse the stone up the urinary tract. This investigation was designed to quantify the last quality for the holmium:YAG laser. The mechanism of action of the pulsed Ho:YAG laser (wavelength 2100 nm) is the generation of a gas plasma at the stone-fluid interface causing a shockwave. The holmium laser was employed for lithotripsy of model stones composed of silicate with a ferrous coating. Stones were selected with a mass of 2 mg +/- 0.1 mg. We sequentially investigated three variables: energy (0.6, 0.8, and 1.0 J), frequency (10, 16, and 20 Hz), and fiber diameter (200, 365, 550, and 1000 microm). Ten stone trials were performed with each of the 36 possible combinations of energy, pulse frequency, and fiber diameter. Our model ureter consisted of a clear rigid polymer tube filled with 0.9% saline. The system was closed and permitted intertrial flushing of generated air bubbles. The laser fiber was maintained at constant extension from the ureteroscope, with stones positioned at the fiber tip before each trial. Laser energy was applied for 2 seconds, with maximum and net retropulsion recorded in millimeters. Each measurement series was recorded in a database for paired Student t-tests. Net retropulsion was then compared by statistically holding each of the three variables constant (fiber size constant with power and frequency varying; frequency constant with power and fiber size varying; and power constant with fiber size and frequency varying). Most retropulsion occurred with the 365-microm and 550-microm fibers. Most comminution was also noted with these fiber sizes. There was no statistical correlation between observed retropulsion and efficiency of comminution. This self-contained model for laser lithotripsy allowed us to measure retropulsion accurately. Silicate stones are not chemically similar to human uroliths but are of uniform composition. The irregular surface characteristics are similar to human stones, making them ideal for retropulsion investigations.

Hollow steel tips for reducing distal fiber burn-back during thulium fiber laser lithotripsy

“…[19][20][21][22][23][24] The bubbles are larger than the fiber diameter and can extend from the fiber tip up to ∼2 mm depending on pulse rate and energy. The mechanical shockwaves created by the cavitation bubbles are not a major contributor to stone fragmentation at the 500-μs pulse duration used in this study, because the ablation mechanism is predominantly photothermal.…”

Comparison of holmium:YAG and thulium fiber laser lithotripsy: ablation thresholds, ablation rates, and retropulsion effects

“…13,14 Furthermore, recent Ho:YAG laser lithotripsy retropulsion studies have concluded that the use of lower pulse energies, longer pulse durations, higher pulse rates, and smaller optical fiber diameters is the optimal combination of laser parameters for minimizing stone retropulsion. [15][16][17][18][19][20] A comprehensive study of all of these parameters was beyond the scope of this paper. However, it should be emphasized that, unlike the flashlamppumped Ho:YAG laser, the diode-pumped TFL is an ideal laser for operation within the range of laser parameters listed above, due to the TFL's excellent spatial beam profile that allows use of small-core fiber diameters and its operation at arbitrary pulse durations and pulse rates.…”

Clinical impact of laser-assisted treatment modalities

“…In urological applications, retropulsion is a common but an undesirable side effect of urological lithotripsy [11][12][13]. Investigations were carried out at the mechanism and its parameter dependency [14][15][16].…”