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Optical breakdown in holmium:YAG laser lithotripsy

Optical breakdown produces shock waves and cavitation and is typically observed with the Q-switched lasers. In contrast, the free-running mode holmium:YAG laser lithotripters produce long (>150 μs) laser pulses that were reported to break urinary stones via thermal decomposition of stones and explosive vaporization of interstitial water without optical breakdown. Here, we show evidence of optical breakdown in holmium:YAG laser lithotripsy using fine temporal and spatial resolution of ultrahigh-speed video microscopy. Images were collected with a Shimadzu HPV-X2 camera at rate up to 10 million frames per second. Infrared- and visible-light photodetectors (10-ns rise time) resolved temporal profiles of infrared laser pulses and visible sparks indicative of optical breakdown. Hydrophone measurements showed shock waves produced by optical breakdown and subsequent collapses of vapor bubbles, capable of breaking glass microscope slides with a single laser pulse. Optical breakdown was observed with the laser fiber tip in contact with synthetic and whole surgically retrieved urinary stones. These observations suggest that the previously unappreciated optical breakdown with holmium:YAG lasers can be an additional acoustic mechanism of action in laser lithotripsy, currently the most used treatment modality for urinary stones, and in other laser procedures. [Work supported by NIDDK of NIH under Award No. R43DK129104.]