Frequency threshold for ablation during holmium laser lithotripsy: How high can you go?

PURPOSE We performed in vitro studies to assess the relationship of pulse frequency on stone ablation during contact laser lithotripsy and determine if there is a threshold after which its effect on lithotripsy is limited. METHODS BegoStones were fragmented using a Ho:YAG laser (P120 Moses) and a 230 µm fiber at 0.5J on Long Pulse (LP) and Moses Distance (MD) modes in contact with the stone. The relationship between the number of pulses (1 to 40Hz) on stone crater volume was assessed using 3D confocal microscopy and nonlinear-segmented regression. To simulate a painting technique, we assessed fragmentation(mg/s) at 20, 40, and 60 Hz, with the fiber moving at a speed of 1 and 3 mm/s, respectively. High-speed imaging was used to record ablation. RESULTS When the laser fiber was fixed, after 13.0 (LP) and 15.4 (MD) pulses, greater pulse frequency did not lead to a significant increase in stone crater volume. Fragmentation was greatest at higher frequencies and faster fiber speed. Increasing the frequency from 20 to 60 Hz at 3 mm/s increased fragmentation by 82% and 61% for LP and MD modes, respectively. Using high-speed data, if the laser fiber is moving at 1mm/s, a hypothetical frequency threshold for ablation was calculated to be 52Hz and 61.6Hz for LP and MD modes, respectively. CONCLUSION Increasing the fiber speed increases stone ablation when using high frequency settings. When the fiber is fixed there is a threshold after which increasing the pulse frequency leads to minimal gain in ablation. The exact value for threshold when the fiber is moving needs further study. Our work serves to provide insight for parameter selection and safety of laser lithotripsy for dusting technique.