Chemical decomposition of urinary stones during Holmium laser lithotripsy - Part I: Lack of a photomechanical effect

The Ho:YAG laser commonly used for clinical lithotripsy of urinary stones typically emits 250-μs pulses at a wavelength of 2.12 μm and repetition rates of up to 10 Hz. This pulse duration is longer than the time required for a pressure wave to propagate beyond the optical penetration depth of this wavelength in water. Fast-flash photography was used to study the dynamics of urinary stone fragmentation by the Ho:YAG laser. Stone ablation began approximately 50 μs after the onset of the laser pulse, long before the collapse of the cavitation bubble at about 350 μs. Pressure measurements, made with a PVDF needle-hydrophone and correlated with the fast-flash images, indicated that the peak acoustical transient was less than 2 bars. Regardless of fiber orientation to the stone, no shockwaves were recorded at the beginning of the bubble, and the maximum pressure waves recorded at bubble collapse were approximately 20 bars. However, no fragmentation occurred during or subsequent to the bubble collapse. These measurements indicated that stone ablation was not due to a photomechanical effect.