Engineers have reversed a
decades-long trend of decreasing efficiency in lithotripsy machines by designing
simple modifications to shock wave lenses. The incidence of kidney stones in
the United States has more than doubled during the past two decades, due at
least in part to the expanding waistlines of its citizens. The condition has
also been linked to hot, humid climates and high levels of stress -- a
combination of living environments that seems to have led to a rise in kidney
stone rates of veterans returning home from Iraq and Afghanistan.
Modern lithotripsy machines used to break
apart kidney stones have been declining in efficiency for decades. Duke
engineers have just modified the shock wave lens to improve treatment.
Credit: Image courtesy
of Siemens Healthcare
Duke engineers have devised a way to improve
the efficiency of lithotripsy -- the demolition of kidney stones using focused
shock waves. After decades of research, all it took was cutting a groove near
the perimeter of the shock wave-focusing lens and changing its curvature.
"I've spent more than 20
years investigating the physics and engineering aspects of shock wave
lithotripsy," said Pei Zhong, the Anderson-Rupp Professor of Mechanical
Engineering and Materials Science at Duke University. "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."
The study appears online the week
of March 17, 2014, in the Proceedings
of the National Academy of Sciences.
The incidence of kidney stones in
the United States has more than doubled during the past two decades, due at
least in part to the expanding waistlines السخافات of its citizens.
The condition has also been linked to hot, humid climates and high levels of
stress -- a combination of living environments that seems to have led to a rise
in kidney stone rates of veterans returning home from Iraq and Afghanistan.
During the past two decades,
lithotripter manufacturers introduced multiple changes to their machines.
Rather than having patients submerged in a bath of lukewarm فاتر 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.
The new designs made the devices
more convenient and comfortable to use, but reduced the effectiveness of the
treatment. After years of research, Zhong and his colleagues 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.
"We were presented with the
challenge of engineering a design solution that mitigated these drawbacks
without being too expensive," said Zhong. "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."
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
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 dead 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.
"We feel we have exceeded
expectations in our evaluation of this new lens design, which is based on solid
physics and engineering principles," said Zhong, who expects the new lens
to enter clinical trials in Germany this summer.
"My hope is that this will
be a breaking point demonstrating that effective, interactive collaboration
between academia and industry can really improve the design of lithotripters
that will benefit millions of stone patients worldwide who suffer from this
painful disease," Zhong said. "Our design, in principle, can be
adapted by other manufacturers to improve their machines as well. I would like
to see all lithotripsy machines improved so that urologists can treat stones
more effectively and patients can receive better treatment and feel more
comfortable with the procedure."
Journal
Reference:
1. A. Neisius, N. B. Smith, G.
Sankin, N. J. Kuntz, J. F. Madden, D. E. Fovargue, S. Mitran, M. E. Lipkin, W.
N. Simmons, G. M. Preminger, P. Zhong. Improving the lens design and
performance of a contemporary electromagnetic shock wave lithotripter. Proceedings
of the National Academy of Sciences, 2014; DOI:10.1073/pnas.1319203111
