THE EFFECT OF TREATMENT STRATEGY ON STONE COMMINUTION EFFICIENCY IN SHOCK WAVE LITHOTRIPSY YUFENG ZHOU, FRANKLIN H. COCKS, GLENN M. PREMINGER AND PEI ZHONG* From the Department of Mechanical Engineering and Materials Science (YZ, FHC, PZ) and Duke Comprehensive Kidney Stone Center, Division of Urology (GMP, PZ), Duke University, Durham, North Carolina ABSTRACT Purpose: The comminution of kidney stones in shock wave lithotripsy (SWL) is a dose depen- dent process caused primarily by the combination of 2 fundamental mechanisms, namely stress waves and cavitation. The effect of treatment strategy with emphasis on enhancing the effect of stress waves or cavitation on stone comminution in SWL was investigated. Because vascular injury in SWL is also dose dependent, optimization of the treatment strategy may produce improved stone comminution with decreased tissue injury in SWL. Materials and Methods: Using an in vitro experiment system that mimics stone fragmentation in the renal pelvis spherical BegoStone (Bego USA, Smithfield, Rhode Island) phantoms (diam- eter 10 mm) were exposed to 1,500 shocks at a pulse repetition rate of 1 Hz in an unmodified HM-3 lithotripter (Dornier Medical Systems, Kennesaw, Georgia). The 3 treatment strategies used were increasing output voltage from 18 to 20 and then to 22 kV every 500 shocks with emphasis on enhancing the effect of cavitation on medium fragments (2 to 4 mm) at the final treatment stage, decreasing output voltage from 22 to 20 and then to 18 kV every 500 shocks with emphasis on enhancing the effect of stress waves on large fragments (greater than 4 mm) at the initial treatment stage and maintaining a constant output voltage at 20 kV, as typically used in SWL procedures. Following shock wave exposure the size distribution of fragments was deter- mined by the sequential sieving method. In addition, pressure waveforms at lithotripter focus (F 2 ) produced at different output settings were measured using a fiber optic probe hydrophone. Results: The rate of stone comminution in SWL varied significantly in a dose dependent manner depending on the treatment strategies used. Specifically the comminution efficiencies produced by the 3 strategies after the initial 500 shocks were 30.7%, 59% and 41.9%, respectively. After 1,000 shocks the corresponding comminution efficiencies became similar (60.2%, 68.1% and 66.4%, respec- tively) with no statistically significant differences (p = 0.08). After 1,500 shocks the final comminu- tion efficiency produced by the first strategy was 88.7%, which was better than the corresponding values of 81.2% and 83.5%, respectively, for the other 2 strategies. The difference between the final comminution efficiency of the first and second strategies was statistically significant (p = 0.005). Conclusions: Progressive increase in lithotripter output voltage can produce the best overall stone comminution in vitro. KEY WORDS: kidney, kidney calculi, lithotripsy Although shock wave lithotripsy (SWL) has been used rou- tinely for the treatment of symptomatic renal calculi for almost 2 decades, the treatment procedure is still largely empirical. Besides some anecdotal opinions, no well-defined protocols of SWL have been developed to ensure an effective treatment outcome with minimal adverse tissue injury. On the other hand, clinical and animal studies have shown that tissue injury in SWL, such as hematuria, the formation of diffuse hemorrhage and hematomas, 1, 2 is dose dependent and increases with the total number of shocks delivered 3 and with pulse energy. 4 Similarly the comminution of renal cal- culi in SWL is also a dose dependent process. 5 Therefore, it is plausible that optimizing the treatment strategy (ie different ways of using the same total amount of acoustic energy delivered to the patient) in SWL may lead to more effective stone comminution with decreased tissue injury. Such a hy- pothesis has not been thoroughly evaluated. Disintegration of renal calculi in a lithotripter field is the consequence of dynamic fracture of the stone materials caused by 2 fundamental mechanisms, namely stress wave induced tensile and shear failure (spalling, shear and tear) near the surface and at internal crystalline-matrix interfaces of a kidney stone, 6–8 and cavitation erosion at the exterior surface of the stone caused by the violent collapse of cavita- tion bubbles. 9 –12 Using an experimental system that mimics stone comminution in the renal pelvis the role of stress waves and cavitation in stone comminution during SWL has been investigated. 5 It was shown that stress waves and cavitation have critical roles in the comminution of kidney stones. They act synergistically rather than independently to ensure the effective and successful fragmentation of renal calculi in SWL. Initially stress waves have a much more important role in breaking up kidney stones into distributed pieces than the cavitation erosion mechanism. However, when the size of residual fragments becomes less than half of the compressive wavelength in the stone material, the effectiveness of stress waves, when acting alone, is hindered. In comparison, the collapse of cavitation bubbles produces damage primarily on the surface of the stone (or residual fragments) and conse- quently weakens the structure of the stone material, making Accepted for publication January 2, 2004. Supported by National Institutes of Health Grants RO1-DK52985 and RO1-DK58266. * Correspondence: Department of Mechanical Engineering and Materials Science, Duke University, Box 90300, Durham, North Carolina 27708-0300 (telephone: 919-660-5336; FAX: 919-660-8963; e-mail: pzhong@duke.edu). 0022-5347/04/1721-0349/0 Vol. 172, 349 –354, July 2004 THE JOURNAL OF UROLOGY ® Printed in U.S.A. Copyright © 2004 by AMERICAN UROLOGICAL ASSOCIATION DOI: 10.1097/01.ju.0000132356.97888.8b 349