Technical insights, controversy mark shock wave lithotripsy's evolution

Q. What are key advances in shock wave lithotripsy since its inception in the United States in the 1980s?

Over the last 2 decades, we've gained a much better understanding of the basic science of how shock waves work and what effect they have on tissue and stones. The advances in lithotripsy have been more in terms of how this knowledge affects techniques, rather than any fundamental evolution in the technology itself.

Fortunately, there is usually only a small amount of tissue injury, and it rarely causes clinical concern. But it is a reflection that something is going on inside the kidney.

By treating the kidney initially with shock waves at a very low power setting that is less injurious to the tissue, we can induce the clamping down of the renal vasculature. Then, after a period of a few hundred shock waves, we can return to our standard power setting and can actually reduce the amount of injury that occurs in the kidney. It's a very simple and practical technique.

Q. What else have we learned about shock wave lithotripsy in the last 20 years?

A. The physics of lithotripsy have become much better understood. When lithotripsy was introduced, the physicists and engineers felt that the compressive forces of the shock wave were entirely responsible for the effect on kidney stone fragmentation.

As it turns out, the shock wave has both compressive and tensile, or negative pressure, components. That negative pressure component produces a process called cavitation, where dissolved gaseous nuclei expand into bubbles and collapse. When those bubbles collapse, typically on the surface of the stone or perhaps within renal tissue, they have quite significant destructive properties.

Cavitation bubbles persist for a fair length of time in tissue, in urine, or around the stone, and will potentially shield the next shock wave if you treat at too fast a rate. We published a paper suggesting that as you treat faster and faster, the negative pressure or tensile component of the shock wave is inhibited (J Endourol 2006; 20:607-11). This may be an important factor in the efficiency of shock waves in stone fragmentation.