V03-01: Understanding non-contact laser lithotripsy for dusting – the popcorn effect: A video analysis
The mechanism of stone breakdown and factors affecting fragmentation during non-contact holmium laser lithotripsy (the popcorn effect) at ureteroscopy (URS) are not fully understood. Using an in vitro model, we undertook high speed video analysis to better understand this mechanism and determine the effects of laser settings, calyceal size, flow rate, time, and heat generation on sub-millimeter fragmentation (i.e. dusting).
BegoStones (15:3) of fixed size were pulverized in a test tube calyceal model of various sizes (11, 15, and 19 mm) using a 120 W holmium laser (P120, Lumenis). A 200 μm laser fiber (Flexiva 200, Boston Scientific) was introduced through the working channel of a URS (LithoVue, Boston Scientific) connected to a 3D positioning system. The fiber was placed 1-2 mm away from the stones. Irrigation was provided at a height of 100 cm. Four different settings were assessed (1.0J x 20-40Hz, 0.5J x 40-80Hz). Video analysis was undertaken using a Photron high speed camera (10,000 and 500 frames per second). Experiments to determine optimal laser setting were conducted for 4 minutes (with 30 second intervals) and subsequent experiments were done at 3 minutes utilizing the best setting - assessing impact of calyceal size, flow rate and time. Fragments were sieved and the primary outcome was % fragments
Video analysis revealed that fragmentation occurred when the stones were in contact with the laser fiber and not when the stones collided. Stone movement occurred as a result of pressure waves generated from vapor bubble expansion and collapse. Superior submillimeter fragmentation was achieved using 0.5J x 80Hz. Short pulse width resulted in significantly greater fragmentation than long pulse. Smaller calyceal sizes led to improved motion and fragmentation. A linear increase in fragmentation over time was observed; however, efficiency decreased after 4 minutes. There was a minimal role of flow rate on outcomes. Temperature reached 50 °C after 30 seconds of continuous firing for 40 W settings, but only 45 °C when activated for 10 seconds.
The mechanism of fragmentation during popcorning is by stone to laser fiber interaction. Stone movement is necessary to improve fragmentation efficiency, and smaller calyceal model sizes led to better results. Superior submillimeter fragmentation was achieved using the 40 W setting (0.5 J x 80 Hz). However, 40 W settings generated more heat so the laser should be activated in short bursts to reduce the risk of thermal injury.
Funding: Research Grant from Boston Scientific