A team of mechanical and materials engineers at Duke University, Durham, NC, have devised a way to improve the efficiency of lithotripsy—the crushing of kidney stones using focused shock waves. After decades of research, they say that all it took was cutting a groove near the perimeter of the shock wave-focusing lens and changing its curvature.

Over the past two decades, lithotripter manufacturers have introduced multiple changes to their machines. Rather than having patients submerged in a bath of lukewarm water, newer machines feature a water-filled pouch that transfers the shock wave into the flesh. An electrohydraulic shock wave generator used in the past was replaced by an electromagnetic model that is more powerful, more reliable, and more consistent. While these new designs have made the devices more convenient and comfortable to use, their effectiveness has been reduced, say the researchers, who believe that they have determined why.

The increased power in some third-generation shock wave lithotripters narrowed the wave’s focal width to reduce damage to surrounding tissues. But this power jump also shifted the shock wave’s focal point as much as 20 millimeters toward the device, ironically contributing to efficiency loss and raising the potential for tissue damage. The new electromagnetic shock wave generators also produced a secondary compressive wave that disrupted one of the primary stone-smashing mechanisms, cavitation bubbles.

The Duke team’s solution was to cut a groove near the perimeter of the backside of the lens and change its geometry. This realigned the device’s focal point and optimized the pressure distribution with a broad focal width and lower peak pressure. It also allowed more cavitation bubbles to form around the targeted stone instead of in the surrounding tissue.

In laboratory tests, the researchers sent shock waves through a tank of water and used a fiber optic pressure sensor to ensure the shock wave was focusing on target. They broke apart synthetic stones in a model human kidney and in anesthesized pigs and used a high-speed camera to watch the distribution of cavitation bubbles forming and collapsing—a process that happens too fast for the human eye to see.

The results showed that while the current commercial version reduced 54 percent of the stones into fragments less than two millimeters in diameter, the new version pulverized 89 percent of the stones while also reducing the amount of damage to surrounding tissue. Smaller fragments are more easily passed out of the body and less likely to recur.