The Sharper Edge

David Pacchioli
November 16, 2011

Prostate cancer, while common, is not easy to spot. Confirming a diagnosis requires a biopsy, a painful procedure with a high rate of false negatives. “Because you can’t target a specific lesion,” says Jason Moore, “it’s difficult to get a good sample.”

Jason Moore

Jason Moore

For Moore, an assistant professor of mechanical engineering at Penn State, the answer is a better needle. And that begins with a better understanding of needle-tip geometry.

“The geometry of the cutting tip has a major impact on cutting efficiency,” Moore explains. But because no one had systematically defined that geometry for surgical needles, he had to turn to the world of manufacturing. Adapting terms and knowledge from the voluminous literature on metal cutting—“drill bits, lathe bits, mill bits, these things have been well-studied,” he says—allowed for accurate comparison of different needle designs to see why some perform better than others. This in turn allowed Moore to create a computer model that predicts a given needle&rquo;s cutting force.

His task was complicated by the nature of soft tissue, which, as Moore notes, “doesn’t act like metal. It wants to deflect. When you insert a needle into it you build up force, build up force, build up force—until it breaks through, and the actual cutting process occurs.” Eventually, though, Moore zeroed in on inclination and rake, the two angles that together define the basic geometry of a cutting tool. Inclination angle, he explains, is the angle of the cutting edge, while rake angle is that of the cutting face, “the plane formed by the needle’s thickness. What we discovered is that inclination angle plays a very important role in cutting force. Rake angle doesn’t matter that much.”

diagram of needle edges
Courtesy J.Z. Moore

Regular two-plane symmetric needle and enhanced cutting edge needle.

Higher inclination angles, his model predicted, would require less force to insert—an important benefit for precise cutting. Subsequent testing of multiple needle styles proved the model’s accuracy. Then, based on these results, Moore and his collaborators modified conventional needles to increase their inclination angles. (“We just cut a ‘V’ into the heel of the bevel,” he says.) Sure enough, in tests on cow’s liver, they were able to get better biopsy samples using less force.

Now, Moore says, he’s looking at a related problem: the accurate positioning of the needle inside the body. “This is especially important for procedures like brachytherapy,” he says, “where radiation ‘seeds’“—tiny pellets—“are placed on a tumor to shrink it. If the positioning is just a few millimeters off, it can greatly affect the amount of radiation delivered, and the side effects.”

Again, he says, “you want to use as little force as possible. Otherwise the tissue will start deforming.” It’s also important to minimize the number of insertion attempts, to prevent swelling. To improve precision, Moore says, he’s looking at automated needle insertion, and also exploring vibration as a way to ease a needle into the body. “This is how a mosquito functions,” he says. “It has such a small mouth you don’t feel it biting you at first.” Vibration, he thinks, might make it possible to use needles of smaller gauge, creating less pain and higher placement precision inside the body.

Only recently arrived at Penn State, Moore plans to broaden his research to include other minimally invasive surgical instruments as well as needles. “I’ve always been interested in designing instruments that can actually help people,” he says, “and I like working with doctors. They always have ideas, and interesting problems that need to be solved.”

Jason Moore, Ph.D., is assistant professor of mechanical engineering,

Last Updated November 16, 2011