Chemical decomposition of urinary stones during Holmium-laser lithotripsy. Part II: Evidence for photothermal breakdown

Because of the ≥250 μs pulsewidth emitted by the Ho:YAG laser used in clinical lithotripsy, it is unlikely that stress confinement occurs within the irradiated stones. Experimental data supports a thermal mechanism for Ho:YAG laser stone ablation. Stone fragmentation occurs soon after the onset of the laser pulse, is uncorrelated to cavitation bubble formation or collapse, and is associated with low pressures (cf. part I). The mass-loss of desiccated calcium oxalate monohydrate (COM) stones exposed to 150 J from the Ho:YAG laser in air was 40±12 mg (mean±1 s.d.); for hydrated stones in air was 25±9 mg; and for hydrated stones in water was 17±3 mg, p<.001. These differences indicate that direct absorption of the laser radiation by the stone is required for the most efficient ablation. Lowering the initial temperature of COM or cystine stones also reduced the stone mass-loss following 20 J of delivered laser energy: 2.2±1.1 mg vs 5.2±1.6 mg for COM stones (-80 vs 23 °C), and 0.8±0.4 mg vs 2.2±1.1 mg for cystine stones (-80 vs 23 °C), p≤.05. Finally, chemical analysis of the laser-induced stone fragments revealed the presence of thermal breakdown products: CaCO ₃from COM; free sulfur and cysteine from cystine; Ca ₂O ₇P ₂ from calcium hydrogen phosphate dihydrate, and cyanide from uric acid.