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The authors report faster lithotripsy in a caliceal model using high pulse energy settings ≥1 J vs low pulse energy settings <1 J. At first glance, this finding seems at odds with existing literature that favors using low pulse energy “dusting” settings. 1 On deeper reflection, the data are more nuanced. Ho:YAG lithotripsy fragments stones via a photothermal mechanism. 2 The number of photons delivered into the stone correlates with ablation volume. 1,3,4 A careful read of the Lumenis 120H cohorts showed the ≥1 J cohorts used power settings of 12–38 W (mean 24 W) vs the <1 J cohorts which used power settings of 6–24 W (mean 12 W). In other words, using twice the power achieved twice the fragmentation speed. All other factors being equal, a car drives faster with a V8 vs a four-cylinder engine.
The laser fragmentation strategy that the authors used for the caliceal model was neither V8 vs four cylinder, but “hybrid.” The authors used direct fragmentation of the caliceal stone phantom wherever possible. But if the stone moved away from the fiber, then they used the “popcorn” technique. “Popcorn” technique may be useful when the scope and fiber cannot orient directly onto the stone. But the “popcorn” technique is an inefficient form of direct contact laser fragmentation. 5 The objective is to force the stone to bounce within a calix as if it were an electron in a p-orbital and maximize the number of random trajectories that happen to fall in the line of fire of the laser fiber. Other studies report popcorn is most efficient at 0.5–1.0 J pulse energy, whereas in this study, the authors show greatest efficiency at 1–2 J pulse energy. 6,7 The geometry and volume of the calix, stone mass, and irrigation pressure undoubtedly factor into the stone trajectory, too. I suspect that the conditions that lead to maximal “fly by's” of the stone traveling into line of laser radiant exposure reflect all these factors.
The unanswered question is as follows: what fragmentation speed might have occurred if they had compared the same power settings using different combinations of low pulse energy at high frequency vs high pulse energy at low frequency? They report overall less total energy used for high pulse vs low pulse energy settings. Regrettably, the data shown do not offer specific apples to apples comparison of cohorts that have identical total power with different energy settings. I wonder what the fragmentation outcomes were for the Lumenis 0.2 J at 70 Hz (14 W) vs 1.5 J at 10 Hz (15 W) or Cook 0.5 J at 20 Hz (10 W) vs 2.0 J at 5 Hz (10 W). One study reported no statistical difference in fragmentation speed in an ureteral stone model when same total energy and power settings were used (0.5 J at 40 Hz vs 2 J at 10 Hz). 1 Another study reported a fragmentation advantage of low pulse energy high-frequency settings for ureteral stones but no fragmentation difference of low (0.2 J) vs medium (0.6 J) pulse energy settings for caliceal stones. 8 Notably, in the current study, the authors report retropulsion in their ureteral model experiment, noting increased retropulsion with shorter pulse duration vs longer pulse duration settings. 9,10 It bears emphasis that retropulsion increases as pulse energy increases, too. 1,4 The fact that the authors switched from direct to “popcorn” fragmentation implies that the stone did displace and the authors thereby switched from efficient to less-efficient fragmentation. To make matters more confusing, it is conceivable that enhanced retropulsion in the “popcorn” model of caliceal lithotripsy might be advantageous as it implies more “fly by's” of the stone in front of the fiber.
So, how should a urologist approach optimal power settings? All other factors being equal: (1) higher pulse energy settings translate to larger crater volumes and faster fragmentation than low pulse energy settings. 1,11,12 This lithotripsy speed advantage from high pulse energy is composition-specific and may also be impacted by fiber susceptibility to burnback. 3,10,13,14 But the inference of the current study that higher pulse energy translates to faster lithotripsy is not absolute. Some studies show a sweet spot at 1.0 J pulse energy and increasing higher than 1.0 J risks fiber burnback as well as larger fragment sizes. 1,9,10 Most studies and compositions tested showed fragments increased from 50 μm range at 0.5–1 J pulse energy settings to as high as 1.5 mm range at pulse energies >1 J, with some studies showing fragments >2 mm. 1,9,10 Whether the 2 mm size fragments in this study are clinically relevant is another matter… but when it comes to fragment size… I assume smaller is better. (2) Increased pulse energy settings increase retropulsion. 1,4,6,8 Retropulsion can be minimized by using low pulse energy settings and by using longer pulse duration settings.
My own take on the literature is to use the Ho:YAG laser at the longest pulse duration setting possible on available machines, use pulse energy settings at the lowest pulse energy that shows demonstrable fragmentation (0.2–0.8 J), and raise the frequency as high as possible (a gazillion would be my desired frequency). As my laser does not go to a gazillion Hz, admittedly, I use a frequency of 20 or 50 Hz (the limits on my two machines)—and sometimes I go to higher pulse energies (1.5 J) when I get impatient and want more bang for the buck…