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Shocking kidney stones
By Tim Stoddard
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Robin Cleveland, an ENG associate professor of aerospace and mechanical engineering, is trying to determine how shock waves break up kidney stones, and the results could lead to a procedure that's kinder to the kidneys. Photo by Kalman Zabarsky |
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Robin Cleveland has never had a kidney stone, but he can sympathize with friends, colleagues, and the 1.3 million Americans who develop the excruciatingly painful stones every year. About 10 percent of Americans will have a kidney stone at some point in their lives, Cleveland says, and in most cases, doctors will use shock waves to slay the demons of the urinary tract. Extracorporeal shock wave lithotripsy (ESWL), as the procedure is known, “works beautifully,” he says. “It's completely revolutionized the treatment of kidney stones. But there are growing concerns that doctors have not given enough attention to the fact that shock waves can do some damage to the tissues around a stone.”
With a $210,000 grant from the Whitaker Foundation, Cleveland, an ENG associate professor of aerospace and mechanical engineering, is trying to understand precisely how shock waves break apart kidney stones in ESWL. In the past two decades, physicians and engineers have proposed several mechanisms to explain how lithotripsy works, but “the problem,” he says, “is that nobody's done definitive experiments to show what's dominating the process because we can't see precisely what's happening to the stone inside the body.”
Kidney stones come in a variety of shapes and sizes, but they are all essentially hard nuggets of calcium and other chemicals that crystallize out of urine. Small stones pass on their own, but larger ones require medical intervention.
Developed by a German aircraft company and first used by doctors in 1980, ESWL was almost immediately embraced as a safe, noninvasive alternative to surgery. Previously, surgeons would slice into the kidney to extract a stone or thread an instrument up the urethra to break it apart. There are now three kinds of lithotripters, but the principle of ESWL has remained the same: a device outside of the body generates an acoustic shock wave, which is focused to a fine point by a reflector. Urologists aim the shock wave point at the stone and fire between 1,000 and 4,000 pulses to break it up into smaller fragments that pass out of the body in urine within hours or days.
But from the beginning doctors could see that it was not an entirely benign procedure. Most patients have hematuria, or blood in the urine, for several days following lithotripsy. “The party line has been that lithotripsy causes very little damage, the kidney heals, and everything is fine,” says Cleveland. But sometimes patients develop more serious side effects, such as hematomas, or pools of blood, on the outside of the kidney, which can put pressure on the kidney and impair its function. Still, compared with surgery, the complications are usually minor. “You can get 5 or 10 percent damage with surgery, even with minimally invasive techniques,” he says. “You're still winning by going with shock waves, but there's some damage that had previously been neglected.”
That's spall, folks
Researchers have hypothesized three different mechanisms by which kidney stones crack under pressure: spallation, cavitation, and shear. “If you take a chunk of concrete and whack it on one side with a hammer, you'll often see a little piece fall off the back side of the block.,” Cleveland says. “That's spall.” In the second mechanism, cavitation, shock waves create tiny cavities in the urine around a kidney stone. The cavities grow into bubbles that pop; as they collapse, a jet of water shoots through the bubbles and drills a hole in the surface of the kidney stone. Cavitation is clearly harmful to the kidneys, though. When cavitation bubbles form inside capillaries, they appear to rip the blood vessels apart. Once bleeding begins, pools of blood are conducive to even more severe cavitation bubbles, which can lead to hematomas.
Shear is the most recently proposed mechanism of kidney stone destruction. “When an acoustic wave enters a stone, some of it will start generating shear waves,” Cleveland says. “Most kidney stones grow over many months in little layers, like an onion. If you can put a good strong shear wave through the stone, you can actually rip those boundaries apart by shear forces.”
Cleveland has been studying the early onset of cracks and fissures in whole kidney stones surgically removed from patients who were not good candidates for lithotripsy. He first looks at the stone with a special scanner at the Beth Israel Medical Center, which takes a three-dimensional X ray of the stone. “It looks into kidney stones beautifully,” he says. “We can see all the internal structures, all the layers.” He then rattles the stone with a burst of 25 to 50 shock waves, and scans it again. Repeating this many times, Cleveland has developed a picture of how the stones break apart.
The results so far have been puzzling. “In some cases, we saw the front surface of the stone being gnawed way, just like in cavitation,” he says. “In other cases, we saw beautiful cracks on the back surfaces, identical to what you'd expect to see in spallation. And we also saw cracks running along the axis of the stone, which behavior you'd expect to see from a shear wave. Our conclusion is that there is no one mechanism that dominates lithotripsy.”
That's an important finding, because it might help physicians and engineers develop new and improved shock waves that break up a stone with shear and spallation rather than cavitation, which can grind up the kidney. “If you can get some information about the internal structure of the stone, that will actually give you a clue as to which mechanism is likely to break it up,” Cleveland says. “It's plausible that if you do the right scan, you can get a hint as to whether it's a homogeneous or inhomogeneous stone. That information could then be used by the person running the lithotripter to select the right wave form for that type of stone.”
In the next year, Cleveland will study how different types of stones break up in lithotripsy. The calcium oxalate stones he's been studying are by far the most common in people, but there are a variety of other stones that may respond to shock waves differently. He's also interested in how the shape of shock waves can be modified to enhance spallation and shear while diminishing cavitation. “We don't think there's ever going to be a silver bullet to break up kidney stones,” Cleveland says. “You're going to have to tailor your shock wave to the stone that's within the patient.”
9
January 2004
Boston University
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