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Abstract
hock wave lithotripsy is a superb example of the successful transition of engineering technology into the clinical area. We have outlined the underlying acoustic principles that describe (1) the generation of the shock pulse, (2) focusing, (3) nonlinear distortion, (4) coupling of the shock source to the body, and (5) absorption of sound by the body. The exact mechanisms by which shock waves can damage stones and tissue are still not fully understood, although it is likely that direct stresses and cavitation are dominant in stone fragmentation, and that cavitation is dominant in tissue injury. Improvements in lithotripsy, whether through improved use of existing lithotriptors or through the development of new technologies, are likely to come only from an improved understanding of the acoustics and the physics of this problem.
Most lithotriptors produce a similar type of shock wave, which consists of aleading positive pressure shock front (compressive wave) followed by a negative pressure trough (tensile wave). There is a large range in the amplitude of the shock waves used, with peak positive pressures of 30 to 110 MPa depending on the type of shock source and the power setting.
The intense compressive wave induces mechanical forces inside the stone that may lead to fragmentation, most likely by a spall mechanism. The tensile component of the shock wave is lower amplitude. This negative pressure drives cavitation bubble activity that is critical to stone comminution, but also causes vascular trauma to the kidney.
Various types of shock wave sources and focusing mechanisms have been exploited in lithotripsy. Electromagnetic and electrohydraulic lithotriptors dominate the lithotripsy market today.
The size and dimensions of the focal zone are controlled by diffraction. Typically, electr magnetic lithotriptors have a smaller focal zone than electrohydraulic devices and generate substantially higher peak positive pressures. A focal zone is not necessarily an advantage because patient motion means that the stone can easily spend a significant amount of time outside the focal region.
Shock waves are coupled into the body using a water path that, ideally, is devoid of bubbles. Most current lithotriptors use an enclosed water path in which the shock head is capped by a rubber membrane of low acoustic impedance. Such dry lithotriptors tend not to be as efficient as the older water-bath lithotriptors in which the patient is immersed in water during treatment—and this reduced efficiency could be due, at least in part, to poorer coupling.
The acoustic waveforms measured in vitro and in vivo are very similar, despite the presence of absorption and heterogeneity in tissue. This is a significant finding for it validates in vitro experimentation as being representative of the in vivo condition.
Numerous mechanisms have been proposed to explain how shock waves break urinary stones. No single mechanism gives an adequate explanation, and it appears that multiple mechanisms involving cavitation and spallation are at play. For tissue injury on the other hand it appears that cavitation, and shock wave/bubble interaction, are the most likely cause of trauma.
Since its inception, lithotripsy has undergone a fascinating evolution. Water bath type, electrohydraulic devices have given way to modular, highly portable lithotriptors, many of which employ electromagnetic shock wave generators. Most lithotripsies are now performed using mobile units delivered by truck to subscribing hospitals. This improved convenience has come at a price as stone re-treatment rates have increased and reports of collateral damage are on the rise. One explanation is that the newer lithotriptors are not as efficacious and have the potential to cause more collateral damage.
New technologies of shock wave delivery are now being applied to patient treatment. Dualpulse lithotripsy uses two shock heads to fire separate pulses. In theory, it should be possible to treat patients faster, and the potential for control over the properties of the acoustic field could lead to improved efficacy and safety. Likewise, initial success with a new lithotriptor that produces a very broad focal zone and is operated at low peak positive pressures suggests that a return to some of the features of the original lithotriptor could also be a step toward improved lithotripsy.