V01-04: High-speed video microscopy investigations of the mechanisms of stone breakage with Holmium:YAG laser lithotripsy
Ureteroscopic laser lithotripsy most commonly uses a Holmium:YAG (Ho:YAG) laser to achieve urinary stone comminution. Stone erosion and fragmentation occurs through photothermal and cavitation-mediated mechanisms. To better understand the process of stone comminution in Ho:YAG laser lithotripsy, stone breakage was investigated in an in vitro model using high-speed video microscopy.
Surgically retrieved whole urinary calculi (calcium oxalate monohydrate) and model (synthetic hydroxyapatite pellets) stones were pulsed with a Ho:YAG laser delivering single shots at energies ranging from 0.2 to 1 Joules. Video images were captured at ~250,000 fps using a high-speed camera (Shimadzu HPV-X2, Kyoto, Japan ) mounted to a Nikon microscope. To better distinguish different mechanisms of stone fragmentation, laser pulses were delivered to dry stones in air and those hydrated in water .
In air, high-speed videography revealed direct light–stone interactions on the stone surface producing photothermal melting, vaporization, and micro-explosions. After several initial (1-20) shots micro-explosions were no longer observed. Termination of the micro-explosions was weakly dependent on the distance between the stone surface and the laser tip. Increasing the laser energy however, restored the occurrence of micro-explosions for additional laser energy delivery before stalling a second time. Additionally, moving the fiber to a new position also restored micro-explosions, producing substantial breakage for the initial (1-20) shots. In water, laser pulses produced a vapor bubble that collapses on the surface of the stone concomitantly with light-stone interactions. Hydrophone measurements suggest this vapor bubble can collapse with sufficient pressure to contribute to stone fragmentation and erosion. Consistent with this finding, direct action of laser light in water upon the stone surface sustained erosion with stone breakage after the initial several shots.
These in vitro findings provide important insights on different mechanisms of stone fragmentation and their interplay during Ho:YAG laser lithotripsy. The direct action of laser light on the stone surface produces substantial breakage that was observed to dramatically diminish in subsequent laser pulses at the same energy and fiber position. This study also suggests that cavitation bubbles contribute to fragmentation and erosion of urinary stones and may also sustain the direct action of laser light on the stone surface in subsequent laser pulses.
Funding: Applaud Medical, Inc.