Photothermal ablation is the primary mechanism in Holmium:YAG laser lithotripsy of urinary calculi

Because of the greater than or equal to 250 greater than or equal to s 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. Previous work has shown that stone fragmentation occurs soon after the onset of the laser pulse, is uncorrelated to cavitation bubble formation or collapse, and is associated with row pressures. Moreover, lithotripsy proceeds fastest with desiccated stones in air (data based on laser ablation of calcium oxalate monohydrate stones), indicating that direct absorption of the laser radiation by the stone material is required for the most efficient ablation. Lowering the initial temperature of calculi reduces the stone mass-loss following 20 J of delivered laser energy: 2.2 +/- 1.1 mg vs 5.2 +/- 1.6 mg for calcium oxalate monohydrate (COM) stones (-80 vs 23 degrees C), and 0.8 +/- 0.4 mg vs 2.2; 1.1 mg for cystine stones (-80 vs 23 degrees C), p less than or equal to.05. In all of the stone compositions examined, thermal breakdown products have been detected, e.g. CaCO3, from COM; free sulfur and cysteine from cystine; Ca2O7P2 from calcium hydrogen phosphate dihydrate, and cyanide and alloxan from uric acid. All of these observations are most consistent with a photothermal breakdown process induced by Ho:YAG laser Lithotripsy.