Evaluation of Retropulsion Caused by Holmium:YAG Laser with Various Power Settings and Fibers
White M, Moran M, Calvano C, Borharn-Manesh A, Mehlhaff B

The ideal intracorporeal lithotriptor 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 inex-pensive, 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 of 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 micrometers). 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 intertribal flushing of gen-erated air bubbles. The laser fiber was maintained at constant extension from the ureteroscope, with stones posi-tioned at the fiber tip before each trial. Laser energy was applied for 2 seconds, with maximum and net retropul-sion 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 micrometer and 550 micrometer 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 composi-tion. The irregular surface characteristics are similar to human stones, making them ideal for retropulsion investi-gations.

Journal of Endourology, Vol. 12, Number 2, 183-186, Apr 1998

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