The authors have to be congratulated to their research concerning the impact of coupling on the results of extracorporeal shock wave lithotripsy. Since the article of Pishchalnikov , the impact of coupling has been extensively studied. Coupling of shock waves depends on the viscosity of the gel, the quality and amount of the gel, and the surface of the skin. Optimal coupling should include large amount of warm ultrasound gel coming from a stock-container rather than from a squeezed bottle to avoid any air pockets with the gel . Indeed, the superiority of the Dornier HM3 regarding clinical success and retreatment rates may predominantly relate to the fact that in this device employs a complete immersion water bath . However, we will not turn the clock backwards, and all future lithotripters will be equipped with dry coupling .
Under this scenario, the study of Li and co-workers reveals important further information: In addition to blocking the transmission of shock wave energy, coupling defects also disrupt the properties of shock waves including focal width and symmetry of the acoustic field. These data underline the importance of optimal coupling eliminating air pockets from the interface. In the study of Bohris , monitoring of coupling with a surveillance camera revealed imperfect coupling in 67%. Basically, in-line ultrasound could be used to check optimal coupling during the procedure particularly focussing on defects located near the centre of the coupling window .