During recent years, much work has focused on breaking urinary and biliary stones using extracorporeal shock wave devices. Up to now, in terms of fragmentation efficacy, the ‘‘electrohydraulic’’ principle is the gold standard. In an electrohydraulic shock wave generator, a plasma is created by a high-voltage electrical discharge between two underwater electrodes. This kind of shock wave generator has a significant drawback because of the large variations of pressure pulses. Moreover, the wear of the electrodes is considerable, leading to nonconstant electrical discharge conditions all along the treatment. A new method to canalize the plasma between the electrodes, using highly conductive liquid, was investigated. Based on this principle a new spark plug was designed. This paper deals with the description and the characterization of this new type of spark plug. The electrical and geometrical parameters are presented. Measurements of the focal point pressure amplitude, of the spatial pressure distribution using different voltage settings, are reported. The acoustic energy in the focal zone is computed. We show that using this principle, the wear of the electrodes has been tremendously reduced. A comparison of in vitro results with other shock wave technologies shows that this new spark plug represents a significant breakthrough in terms of stone fragmentation efficacy. This spark plug is not only useful in the medical field, but it should also be of great interest to laboratories.

The electroconductive lithotripter (ECL) is a new concept for shockwave generation in which a highly conductive solution channels the discharge between the anode and cathode. In vitro experiments showed a linear relation between the voltage setting and the pressure at F2. In vitro stone disintegration studies showed a considerable reduction in shockwave pressure variability, improved energy transfer to the stone, and a unique linear relation between fragmentation and electrode voltage without a saturation effect. This new concept has been used clinically in the Sonolith 4000 lithotripter. In 142 evaluable treatments with a 3-month follow-up, the overall stone-free rate was 82%, and the retreatment rate in stone-free patients was 10%. For stones equal or less than 10 mm, the 3-month stone-free rate, retreatment rate, and secondary procedure rate were 85%, 5%, and 0%, respectively. For stones between 11 and 20 mm, these figures were 83%, 4%, and 2%, respectively. The efficiency quotient was found to be 81% for stones equal or less than 10 mm and 78% for stones between 11 and 20 mm. These clinical results confirm the improvements in efficacy observed in vitro with very satisfactory tolerance.

O INSERM Unitk 281,151, Cows Albert Thomas, 69424 LYONCedex 03, France. O0 Technomed, 103, rue du Dr. Roux, Parc Club du Moulin h Vent, ABSTRACTIn an electrohydraulic generator two underwater metal electrodes are connected in series with a capacitor charged to a high voltage. When the circuit is switched on, a plasma occurs reaching temperatures of thousands of degrees Celsius resulting in a compressive pressure pulse. The formation of the plasma is a particular non reproducible phenomena inducing great variations of the pressure pulse values. Measures obtained by triangulation show that the birth place of the plasma is changing shock after shock and that the center of the spheric divergent shock wave can be located largely outside the inter-electrode gap when tap water is used. In opposite the center of the shock wave is always located at the same place when electrolyte is used. Photographies confirm qualitatively this phenomena. We also show that using electrolyte instead of tap water the mean pressure value is increased by 50 % and that the standard deviation is reduced from 30 % to 5 %.The reproducibility and the higher value of the pressure wave would allow to reduce analgesia and/or the length of the treatment.

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