Scanning electron microscopy of real and artificial kidney stones before and after Thulium fiber laser ablation in air and water

Abstract

We investigated proposed mechanisms of laser lithotripsy, specifically for the novel, experimental Thulium fiber laser (TFL). Previous lithotripsy studies with the conventional Holmium:YAG laser noted a primary photothermal mechanism (vaporization). Our hypothesis is that an additional mechanical effect (fragmentation) occurs due to vaporization of water in stone material from high absorption of energy, called micro-explosions. The TFL irradiated calcium oxalate monohydrate (COM) and uric acid (UA) stones, as well as artificial stones (Ultracal30 and BegoStone), in air and water environments. TFL energy was varied to determine the relative effect on the ablation mechanism. Scanning electron microscopy (SEM) was used to study qualitative and characteristic changes in surface topography with correlation to presumed ablation mechanisms. Laser irradiation of stones in air produced charring and melting of the stone surface consistent with a photothermal effect and minimal fragmentation, suggesting no mechanical effect from micro-explosions. For COM stones ablated in water, there was prominent fragmentation in addition to recognized photothermal effects, supporting dual mechanisms during TFL lithotripsy. For UA stones, there were minimal photothermal effects, and dominant effects were mechanical. By increasing TFL pulse energy, a greater mechanical effect was demonstrated for both stone types. For artificial stones, there was no significant evidence of mechanical effects. TFL laser lithotripsy relies on two prominent mechanisms for stone ablation, photothermal and mechanical. Water is necessary for the mechanical effect which can be augmented by increasing pulse energy. Artificial stones may not provide a predictive model for mechanical effects during laser lithotripsy.