Editorial Comment on: “The Role of Cavitation in Energy Delivery and Stone Damage During Laser Lithotripsy” by Ho et al.

Initial clinical lasers for lithotripsy (the pulsed dye laser) were characterized by short pulse durations (<2 microseconds) that produced a rapidly expanding spherical cavitation bubble. Spherical cavitation bubbles collapse to a single locus with an outsized collapse pressure (proportional to the maximal bubble expansion radius raised to the power of 3). This outsized collapse pressure produces photomechanical (or photoacoustic) lithotripsy. ^1,2

The subsequent introduction of the holmium:Yttrium-Aluminium-Garnet (YAG) laser with its long pulse (typically an optical pulse duration of 175 to 300 microseconds) proved a game changer. Holmium (with a pulse duration of several hundred microseconds) works by a photothermal mechanism whereby photons are absorbed directly by the stone. ³ The long pulse also creates an asymmetric bubble that collapses weakly to several loci without significant outsized collapse pressure of pulse lasers of pulse durations <2 microseconds. The photothermal mechanism explains why Ho:YAG laser fragments stones better than prior devices: Ho:YAG laser fragments all compositions, produces smaller fragments, and creates less retropulsion. ^4,5 Photons delivered into the stone translate roughly to ablation plume ejected off the stone. ⁶

The authors describe an ingenious set of experiments that demonstrates a more complicated state of affairs for long pulse Ho:YAG lithotripsy. ⁷ They show photothermal ablation with the fiber fixed in perpendicular orientation to the stone surface (0° laser incidence) with a law of diminishing ablation returns after 30–40 pulses. This observation makes sense: as the ablation crater deepens with repeat pulses, the separation distance increases and more photons are subsequently absorbed by the intervening water with fewer photons reaching the stone, so less ablation occurs. ⁸ But with the fiber in parallel orientation (90° laser incidence), no photons should reach the stone and no ablation should occur if a purely photothermal mechanism were the full story. However, they show with increasing pulses in this parallel orientation, ablation does occur and increases with the pulse count. The mechanism of ablation appears to be water jets directed toward the stone. They show a sweet spot at 1 mm separation for the experimental conditions tested. (Water jet-induced ablation requires a Goldilocks scenario of the bubble collapse to be at just the right distance with too close not giving much ablation at all and too far yielding no ablation). ^{9 –11}

I congratulate the authors for this outstanding scientific work. It alters the simple photothermal paradigm and shows that long pulse Ho:YAG lithotripsy might be dominated by photothermal ablation, but with the right conditions present some mechanical effects of the cavitation bubble collapse might contribute to additional lithotripsy. ⁹

My take-home points are (1) if you are using typical dusting settings (0.2 J at 70 Hz) with the scope and fiber held fixed in relation to the stone surface, you will achieve deepening ablation craters quickly. Within 1 second as the ablation crater deepens, photons will be absorbed by the intervening irrigation and not reach the stone. The clinical inference is to “paint the stone surface” by moving the laser constantly if using high-frequency settings and (2) the water jet phenomenon in near boundary conditions requires more data to know how best to use it for safe ablation. I do not recommend urologists park the fiber in parallel orientation to the stone hoping for lithotripsy when you risk targeting soft tissue. Further studies may elucidate how to use water jet-induced fragmentation to advantage.