Editorial Comment: Elucidating the Mechanism of Stone Dusting Requires a Fresh and Rigorous Approach in the New Era of Laser Lithotripsy

Reply to Editorial Comment:

We would like to first acknowledge the significant contribution of the reviewers for their extensive work in laser lithotripsy (LL), which has established the photothermal ablation theory of stone damage, ¹ and for writing this editorial comment. ²

Advances in laser technologies in recent years, however, have significantly altered the landscape and clinical mode of operation in LL for the surgical management of urinary stone disease. ^3,4 The predominant mode of LL has shifted gradually from the conventional fragmenting mode with basket extraction to in situ dusting mode without the need for fragment removal. Concomitantly, the laser parameters employed by Ho:YAG lasers have also changed drastically from high energy (0.8–1.2 J)/low frequency (typically <5 Hz) for fragmenting to low energy (0.2–0.4 J)/high frequency (40–120 Hz) for dusting.

It is important to note that the clinical success for dusting, unlike fragmentation, is not critically dependent on keeping the laser fiber in contact with the stone. In fact, it is difficult to keep the fiber tip in contact with the stone during the entirety of a procedure and, therefore, slight noncontact or “defocusing” of the laser is often preferred during dusting. ⁵

These fundamental changes in laser technology and techniques require a fresh re-evaluation of the mechanism of action in LL, especially with the emerging evidence that we and others have demonstrated recently regarding the role of cavitation in stone ablation. ^{6 –9} The need for more rigorous research in LL is further driven by the rapid introduction of new laser systems (both Ho:YAG lasers and thulium fiber lasers) on the market with less appreciation of the optimal conditions under which these devices should be operated. Without this understanding, we not only run the risk of reduced efficacy but also of potential patient harm. ⁹

Because of the shallow penetration depth of Ho:YAG laser in water (∼0.4 mm), physically, a higher percentage of the laser pulse energy in dusting will be absorbed by the interposing fluid (or scattered by the dust) with resultant less energy delivered to the stone material. ⁶ As a result, the conditions typically conducive for photothermal ablation during fragmenting are likely to diminish during stone dusting, especially using clinically advocated noncontact mode (e.g., fiber to stone distance <0.5 mm) and at high frequencies.

Concurrently, most of the laser energy deposited in the fluid will lead to bubble formation, expansion, and collapse, as well as temperature rise in the urinary space. ^10,11 It is, therefore, imperative to understand more precisely the laser–fluid–bubble–stone interaction so that we will be able to harvest the potential of cavitation energy to improve treatment outcome while reducing adverse effects during LL. Such knowledge is currently lacking in the literature.

The data we presented ⁶ are thought provoking and in distinct contradiction to the conventional photothermal ablation theory of stone damage. ¹ Yet, our observations remain preliminary in terms of understanding the complex physical processes involved in LL. As elegantly suggested by Teichman and colleagues, ² more work needs to be done using human stones of various compositions, under clinically relevant conditions, such as scanning the fiber tip over the stone surface, and with long pulsed lasers.

Nevertheless, the bubble dynamics and predominant mechanism of stone dusting are likely to vary with the laser wavelength, pulse profile, duration, and delivery frequency, ^{12 –14} all of which raise the question regarding the optimal design of laser pulses and operational conditions for dusting. Understanding such a complex procedure in LL is challenging, yet essential for gaining physical insights into the mechanism of action, from which we may devise better treatment strategies to improve clinical outcome and safety.

Looking forward, this challenge also presents a unique opportunity for promoting research collaborations from different groups in the field in terms of standardization of stone phantoms, kidney models, animal studies, and clinical protocols for LL research and applications. Together with the support of NIH and medical device industry, such a cohesive effort will help to optimize the design of laser systems and clinical strategies for most effective and efficient energy delivery and utilization to treat stone patients with minimal adverse effects, procedure time, and costs.