Contact or Noncontact Laser Lithotripsy? (From: Tracey J,Gagin G,Morhardt D,et al. J Endourol 2018;32:290–295)

Dear Editor:

With great interest, we read the article of Tracey et al. ¹ studying ureteroscopic high-frequency dusting utilizing a 120-W Ho:YAG laser generator. Although not all stones were amenable to dusting, the authors reported acceptable residual fragments and complication rates by using laser energies of 0.2–0.3 J and a frequency of 70–80 Hz. They also noted that high-frequency dusting has the advantage of a diminishing need of ureteral access sheaths, since stone basketing and its need for multiple instrument insertions can be avoided. That said, omitting the use of ureteral access sheaths (UASs) should be put in balance with the impact on irrigation flow, since continuous evacuation of dust appears to be crucial to ensure acceptable visibility during high-frequency dusting.

In our opinion, another important factor deserves particular attention and may surpass the proposed advantages of high-frequency dusting. To date, studies regarding laser lithotripsy have mainly focused on fragmentation efficiency by changing lithotripter settings and laser fiber diameters. Increasing pulse energy, pulse frequency, or using short pulse durations results in increased stone ablation efficacy, retropulsion, and fiber tip degradation. ^2,3 Surprisingly, no study examining how the distance between the laser fiber and the stone interacts with fragmentation efficiency and in fiber tip degradation has been conducted yet.

Of interest, currently available studies on laser efficiency were performed with the laser fiber in direct contact with the stone surface. It is supposed that direct contact minimizes the formation of a cavitation bubble by absorption of laser radiation in the liquid. When the distance between the laser fiber and the stone increases, more pulse energy may be consumed for the cavitation effect and only a small amount of pulse energy may remain for ablation. ⁴

The reports by Chawla and coworkers ⁵ and Emiliani and coworkers ⁶ are the sole studies that assessed the validity of noncontact Ho:YAG laser stone fragmentation in an in vitro model. They found that this method was effective for a high-energy, high-frequency, and long pulse duration, defining this concept as the “popcorn effect.” Noncontact fragmentation may also be of interest for lower calix stones that are difficult to reach or stones that are not in the direct line of sight because of limitations of endoscope deflection.

Better understanding of the effect of distance variation between the laser fiber tip and the stone surface is crucial, since the mechanism of noncontact Ho:YAG laser stone fragmentation is unclear. Also, it is essential to understand how that distance stands in relation to various laser settings to achieve optimal efficiency and prevent fiber degradation. If it turns out that the ideal delivery of pulse energy occurs at a defined noncontact distance to stone, a laser fiber with a spacer between the stone surface and the fiber tip may be developed to fix the ideal distance to stone. This may lead to improved practice and treatment strategies of endourologic stone management.