EVALUATION OF RETROPULSION CAUSED BY HOLMIUM-YAG LASER WITH VARIOUS POWER SETTINGS AND FIBERS

The ideal intracorporeal lithotriper would comminute all types of calc uli into small readily excreted particles. It would be small and flexi ble with an energy source safe for the uroepithelium. It should not br eak, should be inexpensive, and should not retropulse the stone up the urinary tract. This investigation was designed to quantify the last q uality for the holmium:YAG laser. The mechanism of action of the pulse d Ho:YAG laser (wavelength 2100 nm) is the generation of a gas plasma at the stone-fluid interface causing a shockwave. The holmium laser wa s 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, 55 0, and 1000 mu m) 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 ext ension from the ureteroscope, with stones positioned at the fiber tip before each trial. Laser energy was applied for 2 seconds, with maximu m and net retropulsion recorded in millimeters. Each measurement serie s was recorded in a database for paired Student t-tests. Net retropuls ion was then compared by statistically holding each of the three varia bles constant (fiber size constant with power and frequency varying; f requency constant with power and fiber size varying; and power constan t with fiber size and frequency varying). Most retropulsion occurred w ith the 365-mu m and 550-mu m fibers. Most comminution was also noted with these fiber sizes. There was no statistical correlation between o bserved retropulsion and efficiency of comminution. This self-containe d model for laser lithotripsy allowed us to measure retropulsion accur ately. Silicate stones are not chemically similar to human uroliths bu t are of uniform composition. The irregular surface characteristics ar e similar to human stones, making them ideal for retropulsion investig ations.