Upgrading Kidney Stone Treatment
Robin Cleveland, an associate professor of aerospace and mechanical engineering in the College of Engineering, uses an electrical device to intervene in a painful biological process — the passing of enlarged kidney stones. Since 1984, American physicians have routinely broken up kidney stones too large to pass easily by firing thousands of pulses at them with an electric-powered acoustic shock wave device called a lithotripter. While the treatment is noninvasive and typically has minimal side effects, scientists have determined that it occasionally causes significant soft tissue damage in the kidney, with side effects including kidney failure and hypertension.
“Even after 20 years of study, we still don’t have exact mechanisms on how the shock waves break up stones and cause side effects,” Cleveland says. “This makes it difficult for lithotripter designers to improve them.”
Now, Cleveland and a network of other collaborators funded by the National Institutes of Health are educating physicians about more effective lithotripter techniques aimed at improving fragmentation efficiency and decreasing tissue injury. These techniques include reducing shock wave delivery rates and using fewer shock waves at lower settings. In the long term, the researchers want to provide guidelines on what shock wave forms will result in good fragmentation and reduced tissue damage. “The ultimate goal would be to take a CT image of a stone,” Cleveland says, “and specify a designer shock wave form that would efficiently fragment a stone while leaving the surrounding soft tissue intact.”
To better understand how shock waves pass through kidney tissue and break kidney stones, Cleveland trains electric-powered lithotripters on artificial and human stones and animal kidneys in a large tank. One such device, an electrohydraulic lithotripter, sends 20,000 volts across the two tips of an electrode, producing a big electrical spark that generates an acoustic shock wave underwater. The electrode is placed within an ellipsoidal reflector that focuses the sound waves to where the kidney stone is positioned.
Cleveland’s research recently revealed the prominent role of shear waves — a type of seismic wave that passes through the body — in the fragmentation of artificial stones. When the incoming shock waves pass into the stone, they generate two kinds of waves in its interior: compression waves (similar to sound waves) and transverse waves (similar to the waves that appear when a person snaps a rope or string). The shear waves increase as they move through the stone and generate larger destructive forces than the acoustic waves. “We identified this with our numerical model and confirmed it with experiments using artificial stones,” says Cleveland. “The next step is to apply the same model to human kidney stones.”
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This article originally appeared in Boston University’s Research 2007 magazine. Click here to read more from the magazine.