Introduction: Ureteroscopic laser lithotripsy is commonly used to break urinary stones but its physical mechanisms are still debated. The dominant mechanism was reported to be the photothermal decomposition of stones, but recent studies also suggest that stones break by the explosive vaporization of interstitial water. Photoionization and plasma formation were also suggested, but questioned, arguing that laser intensities are insufficient to produce optical breakdown. Cavitation mechanism was reported to be negligible, but recent studies suggest that it plays a role in stone breakage. The objective of this study was to better understand the mechanisms of laser lithotripsy by using ultrahigh-speed video microscopy of stone breakage in air and water.
Methods: Holmium:YAG laser pulses at 0.2–2.0 J were used to fragment synthetic hydroxyapatite and surgically retrieved urinary calcium oxalate monohydrate stones. The dynamics of stone breakage was recorded via a Nikon TS100 microscope with a Shimadzu HPV-X2 camera at frame rate of 200,000–1,000,000 frames per second. For quantitative assessment of stone fragmentation, the images were processed in Python using Open-source library of Computer Vision (OpenCV) algorithms. Stone fragments were identified by detecting the difference between the images and tracked considering velocity and size of the fragments. The number and cross-sectional area of the fragments were used as a metric of stone fragmentation.
Results: Stones in air fragmented with microexplosions. Depending on experimental conditions—such as laser energy, stone content, wet versus dry—the microexplosions were driven by photothermal decomposition, vaporization of interstitial water, photoionization and plasma expansion. Microexplosions diminished with subsequent laser pulses (P <0.001) and could be resumed by either moving the fiber to an untreated stone position or increasing the laser energy. Microexplosions were also resumed by wetting the dry stones. In addition to microexplosions, cavitation was another mechanism contributing to stone breakage in water.
Conclusions: The observed microexplosions unify previously suggested mechanisms of stone breakage encompassing various underlying physical processes: melting and vaporization of stone material, ionization and plasma expansion, and vaporization of interstitial water. Further exploration of microexplosions and cavitation will aid in development of future improvements of laser lithotripsy.