How Lasers Ablate Stones: In-vitro Study of Laser Lithotripsy (Ho:YAG and Tm-fiber lasers) in Different Environments.

There are two main mechanisms of stone ablation: photothermal and thermomechanical. Which of them is primary in stone destruction is still a matter of discussion. Water holds importance in both mechanisms but plays a major role in the latter. We sought to identify the prevailing mechanism of stone ablation by evaluating the stone mass-loss after lithotripsy in different media.We tested a Ho:YAG laser (100W, Lumenis, USA), a Tm-fiber laser U1 (TFL U1), (120W, NTO IRE-Polus, Russia) and a Superpulse Tm-fiber laser U2 (TFL U2) (500W, NTO IRE-Polus, Russia). A single set of laser parameters (15 W = 0.5 J × 30 Hz) was used. Contact lithotripsy was performed in phantoms (BegoStones) in different settings: a) hydrated phantoms in water; b) hydrated phantoms in air; с) dehydrated phantoms in water; d) dehydrated phantoms in air. Laser ablation was performed with total energy of 0.3 kJ. Phantom mass-loss was defined as the difference between the initial phantom mass and the final phantom mass of the ablated phantoms.For all laser groups, hydrated phantoms ablated in water demonstrated the largest mass-loss (up to 61.3 ± 7.8 mg in TFL U2). The ablation of dehydrated phantoms in air exhibited the least mass-loss in HoYAG and TFL U2 groups (7.5 ± 0.7 and 3.0 ± 0.1 mg), while the least mass-loss (11.3 ± 1.4 mg) after TFL U1 exposure was seen in dehydrated phantoms ablated in water. Among laser types, TFL U2 had the highest phantom mass-loss in all groups except for dehydrated phantoms ablated in air (3.0 ± 0.1 mg).Our results suggest that both photothermal and thermomechanical ablation mechanisms occur in parallel during laser lithotripsy. In Ho:YAG and TFL U2 stone ablation, thermomechanical mechanism prevail, while in TFL U1 ablation, photothermal mechanism appear to predominate.