Researcher improves needle-tip geometry for better biopsies

November 28, 2011

UNIVERSITY PARK, Pa. -- A better understanding of needle-tip geometry could lead to more accurate and less painful biopsies used to confirm prostate cancer, says a Penn State researcher.

Currently, such biopsies can be painful procedures with a high rate of false negatives.

"Because you can't target a specific lesion, it's difficult to get a good sample," said Jason Moore, assistant professor of mechanical engineering. The answer is a better needle.

"The geometry of the cutting tip has a major impact on cutting efficiency," he said.

Because no one had systematically defined that geometry for surgical needles, Moore turned to the world of manufacturing. Adapting terms and knowledge from the voluminous literature on metal cutting allowed for accurate comparison of different needle designs to see why some perform better than others. This allowed Moore to create a computer model that predicts a given needle's cutting force.

His task was complicated by the nature of soft tissue.

"It doesn't act like metal," he said. "It wants to deflect. When you insert a needle into it, you build up force until it breaks through, and the actual cutting process occurs."

Eventually, Moore zeroed in on inclination and rake, the two angles that together define the basic geometry of a cutting tool. Inclination angle is the angle of the cutting edge, while the rake angle is that of the cutting face, or 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," Moore said.

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. Based on these results, Moore and his collaborators modified conventional needles to increase their inclination angles. In tests on cow's liver, they were able to get better biopsy samples using less force.

Moore also is investigating the accurate positioning of the needle inside the body, a related problem.

This is especially important for procedures where radiation 'seeds' or tiny pellets are placed on a tumor to shrink it," he said. "If the positioning is just a few millimeters off, it can greatly affect the amount of radiation delivered, and the side effects."

As little force as possible should be used. Otherwise the tissue will start to deform. It's also important to minimize the number of insertion attempts to prevent swelling.

To improve precision, Moore is researching automated needle insertion and also exploring vibration as a way to ease a needle into the body.

"This is how a mosquito functions," he said. "It has such a small mouth you don't feel it biting you at first."

Vibration might make it possible to use needles of smaller gauge, creating less pain and higher placement precision inside the body.

  • Regular two-plane symmetric needles (left) and enhanced cutting edge needles (right), tested for biopsy.

    IMAGE: J. Z. Moore
Last Updated December 01, 2011