Frequency Threshold for Ablation During Holmium Laser Lithotripsy: How High Can You Go?

Abstract

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 lm fiber at 0.5 J on long pulse (LP) and Moses distance (MD) modes in contact with the stone. The relationship between the number of pulses (1-40 Hz) on stone crater volume was assessed using three-dimensional confocal microscopy and nonlinear-segmented regression. To simulate a painting technique, we assessed fragmentation (mg/second) at 20, 40, and 60 Hz, with the fiber moving at a speed of 1 and 3 mm/second, 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/second increased fragmentation by 82% and 61% for LP and MD modes, respectively. Using high-speed data, if the laser fiber is moving at 1 mm/second, a hypothetical frequency threshold for ablation was calculated to be 52 and 61.6 Hz 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 study serves to provide insight for parameter selection and safety of laser lithotripsy for dusting technique.

The Optimal Combination of Scanning Speed and Fiber-to-Stone Distance for Effective and Efficient Dusting during Holmium: YAG Laser Lithotripsy

“…This feature can be seen from the distinct shift of the optimal v fiber from 3.5 mm/s to 4.1 mm/s under SD = 0.25 mm, and from 0.5 mm/s to 3.5 mm/s under SD = 0.50 mm when the bubble collapse toward the stone surface was eliminated. This observation is consistent with previous studies, in which a faster scanning speed (e.g., 3 mm/s vs. 1 mm/s) was found to produce higher dusting efficiency regardless of PRF in contact mode (13). Under similar settings (0.5 J, 20 Hz) using another laser lithotripter with different pulse profiles, the maximum dusting efficiency could be achieved at even higher v fiber of 10 mm/s (23).…”

“…Conventional theory in the past two decades attributes photothermal ablation and microexplosion to be the dominant mechanisms of stone damage in LL (2,9,10). As a result, previous studies often advocate placing the fiber tip in contact with the stone surface, i.e., SD = 0 mm, with the hope to maximize laser energy transmission and treatment efficiency (11)(12)(13)(14).…”

“…Clinically, however, maintaining a close contact of the fiber tip with the stone surface is challenging during LL. In particular, a moving fiber cannot be placed precisely in constant contact with the stone due to either changing surface curvature of the stone (13), the retropulsion effect (15,16), or respiration-induced kidney movement (5). Moreover, a panel of leading endourologists have recently recommended that dusting efficiency can be improved clinically by “defocusing” the laser beam via pulling back the fiber tip slightly away from the stone surface to produce small fragments (5).…”

“…Several groups have investigated the effect of fiber scanning on stone damage in vitro during Ho:YAG LL, mostly in contact mode (i.e., SD ≈ 0 mm where photothermal effects dominate). At E p = 0.5 J and PRF = 20 Hz, Alhoukhi and colleagues compared two clinically relevant fiber scanning speeds and found that the mass loss was much greater at 3 mm/s than at 1 mm/s (13). Using similar settings, Panthier et al demonstrated that the maximum stone damage could be produced at v fiber = 10 mm/s or 2.5 mm/s, compared to lower scanning speeds (23).…”

New Generation Pulse Modulation in Holmium:YAG Lasers: A Systematic Review of the Literature and Meta-Analysis

“…Aldouki et al [ 23 ] found that MD had the highest ablation and fragmentation rates both at 0 and 1 mm distance when compared to the rest of the Lumenis’ PMs. In another article, the same authors found better stone ablation with MD than with SP, especially at a fiber moving speed of 3 mm/s rather than 1 mm/s [ 25 ].…”