Stone Retropulsion Caused by the Pulse-Duration Adjustable Holmium Laser: Analysis of the Whole-Process Dynamics with a Modified Method

Introduction: Stone retropulsion was shown to be impacted by pulse duration during holmium laser lithotripsy, although the whole process of retropulsion was troublesome to study. We developed a modified method to analyze retropulsion using a smartphone and video tracking software. Materials and Methods: A holmium laser system was incorporated with a short (200 μseconds) and long pulse-duration (LP) (800 μseconds), and a 272-μm core fiber was attached. A cross-sectional V-shaped rail was submerged in a tank, on which artificial stones were displaced linearly after lasering. Different combinations of pulse energy, frequency, and pulse duration were tested for at least 4 seconds. An iPhone 11 capable of high-definition videoing and video tracking software was used to analyze the stone's displacement and velocity. Results: For most settings, the displacement–time graph resembled logarithmic growth and the velocity peaked within the first second after lasering. Higher energy or frequency translated into greater displacement, accompanied by earlier and faster velocity peaks. When the laser power was constant, the short pulse-duration at the fourth second after lasering was much larger in 0.5 J × 40 Hz than 1.0 J × 20 Hz under the short pulse-duration (SP) (13.17 ± 0.92 mm vs 6.90 ± 1.98 mm, p < 0.05), but this discrepancy was offset by the LP. The largest stone displacement and velocity were observed in 0.5 J × 40 Hz SP. Conclusion: The pulse duration plays a dominant role in determining the stone retropulsion and velocity, and a long pulse decreases retropulsion and velocity. Given a constant power, the variable combination of frequency and pulse energy contributes to significantly different retropulsion with a short pulse rather than a long pulse. The modified method offers a feasible solution for the study of stone retropulsion by laser lithotripsy.