Mechanism of urinary stone fragmentation using a holmium yttrium-aluminum-garnet (HO:YAG) laser

A pulsed Ho:YAG laser (wavelength: 2.1 μm) has been clinically used for the urinary stone fragmentation, but optical absorption of stones are weak at 2.1 (ini. To elucidate the mechanism of the stone fragmentation with Ho:YAG laser, we determined the fragmentation efficacy in air or in water. First, stone fragmentation processes and cavitation bubble formation were observed by time-resolved flash photography. Second, the stone model was made as a mixture of plaster and emery powder. The models in dry condition (water content: 14%), in wet condition (44%), or in water were fragmented with Ho:YAG laser at energy level of 0.5 J and at a frequency of 10 Hz via quartz fiber. Third, the various components of stones were fragmented in air or in water similarly. We defined the stone weight, which was fragmented to less than 4 mm in diameter, divided by amount of energy as the fragmentation efficiency. The bubble formation and stone fragmentation were shown on a micro-second time scale. The fragmentation efficiency of models in dry condition, in wet condition and in water was 0.12 mg/J,0.82 mg/J, and 3.0 mg/J, respectively. The fragmentation efficiency in air and water was 0.12 mg/J and 0.33 mg/J for calcium oxalate stone, 0.37 mg/J and 1.1 mg/J for calcium phosphate stone, 0.48 mg/J and 0.63 mg/J for magnesium ammonium phosphate stone, 0.02 mg/J and 0.44 mg/J for uric acid stone, and 0.12 mg/J and 0.15 mg/J for cystine stone. Since the fragmentation efficiency of stone models and various components of stones depends on the water contents, our experiment suggests that the stone fragmentation with pulsed Ho:YAG laser is caused by the expansion of vapor bubble, which is produced by irradiation in water within or around a stone.