Understanding the Popcorn Effect During Holmium Laser Lithotripsy for Dusting

OBJECTIVE To assess low and high power settings for the popcorn technique, and relationship of laser fiber-to-stone distance and calyceal size on submillimeter fragmentation. Our in vitro findings may help guide strategies to improve a dusting technique for ureteroscopy. METHODS BegoStones were fragmented in small (127 mm(3)) and large (411 mm(3)) sized bulbs to simulate calyces, using a 120 W Ho:YAG laser. A 242 mu m fiber was introduced through a ureteroscope mounted to a 3D positioner with its tip located at 0 or 2 mm distance from the stones. 20 W [1 J x 20 Hz, 0.5 J x 40 Hz] and 40 W [1 J x 40 Hz, 0.5 J x 80 Hz] settings were assessed, including short pulse and long pulse modes. Total energy delivered was constant at 7.2 kJ. Primary outcome was percentage of stone mass converted to fragments <1 mm. High-speed imaging was performed to study stone movement and/or fragmentation. RESULTS For all settings, popcorn lithotripsy yielded more submillimeter fragments when performed with the fiber positioned on the stone compared to 2 mm from the stone (P <.05). Distribution of submillimeter fragments was higher when utilizing high frequencies regardless of pulse energy. At 2 mm distance, popcorning was more effective in the small model (P <.05). At 2 mm distance, short pulse was superior to long pulse. Video analysis showed fragmentation did not occur when stones collided with each other. At 80 Hz/2 mm distance, only 17.5% of pulses impacted fragments. CONCLUSION Popcorn technique is more effective when the fiber is directly in contact with stone, and when performed in a small calyceal model. Utilizing settings with higher frequencies may improve dusting outcomes. (C) 2018 Elsevier Inc.