Mechanisms of Stone Fragmentation Produced by Nano Pulse Lithotripsy (NPL)

Abstract

Nano Pulse Lithotripter (NPL) is a new technology in intracorporeal lithotripsy. It utilizes a high-voltage spark discharge of about 30-nanosecond duration, released at the tip of a flexible probe under endoscopic guidance, to break up kidney stones into fine powders for spontaneous discharge. Several primary damage patterns have been observed during the NPL treatment of hard and soft BegoStones: crater formation near the probe tip, crack development from the distal wall of the stone, and crack initiation in the form of radial and ring-shape circumferential fracture in the proximal surface of stones of small sizes. Compared to the traditional intracorporeal lithotripsy technologies (laser and EHL), NPL has been shown to comminute kidney stones with higher efficiency, especially with hard stones, although its working mechanisms are largely unclear. Multiple potential contributory factors have been proposed: direct dielectric breakdown in the stone material near the NPL probe tip, shock wave induced by the spark discharge in the fluid, and cavitation and resultant asymmetric collapse of bubbles. Various experiments have been carried out to correlate each of the proposed mechanisms with the damage patterns observed. Comparison between micro-CT images of the damage initiation sites and COMSOL simulation of the stress field in the stone indicate that the observed cracks are most likely to be produced by the locally intensified tensile stresses associated with the surface acoustic waves (SAW) generated by the incidence of the spark-generated, spherically divergent shock wave on the proximal surface of the stone, and/or their interactions with bulk acoustic waves (P or S) upon reflection from the stone boundaries. Dielectric breakdown is found to potentially contribute to crater formation, especially for soft BegoStones. However, the contribution of cavitation to stone fragmentation in NPL appears to be minimal.