Lens design may improve lithotripter performance

A collaborative effort involving Duke University experts in urology, engineering, pathology, and mathematics has resulted in improved efficiency for shock wave lithotripsy, the researchers report.

New designs introduced to lithotripters over the past 2 decades made the devices more convenient and comfortable to use, but reduced the effectiveness of the treatment. After years of research, Pei Zhong, PhD, MS, Glenn M. Preminger, MD, and colleagues have determined why.

“And now, thanks to the willingness of Siemens [a leading lithotripter manufacturer] to collaborate, we’ve developed a solution that is simple, cost-effective and reliable that can be quickly implemented on their machines,” said Dr. Zhong, the study’s senior author and a mechanical engineer at Duke, Durham, NC.

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 mm 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 researchers reported online in the Proceedings of the National Academy of Sciences (March 17, 2014).

“We were presented with the challenge of engineering a design solution that mitigated these drawbacks without being too expensive. It had to be something that was effective and reliable, but also something that the manufacturer was willing to adopt. So we decided to focus on a new lens design while keeping everything else in their system intact,” Dr. Zhong said.

The 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 authors 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 anesthetized pigs and used a high-speed camera to watch the distribution of cavitation bubbles forming and collapsing.

The results showed that while the current commercial version reduced 54% of the stones into fragments less than 2 mm in diameter, the new version pulverized 89% of the stones while also reducing the amount of damage to surrounding tissue.

“We feel we have exceeded expectations in our evaluation of this new lens design, which is based on solid physics and engineering principles,” said Dr. Zhong, who expects the new lens to enter clinical trials in Germany this summer.

Dr. Zhong, Dr. Preminger, and several of their co-authors hold a patent entitled “Acoustic Lens for Shockwave Lithotripsy and Related Methods” on this technology, which is owned by Duke University.

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