HANCOCK, N.H. — A wireless strain sensor built into an artificial knee replacement is providing medical-prosthesis engineers with their first view of real-time forces inside a human joint during normal activities.

The piezoelectric sensor design was provided by Microstrain Inc. (Williston, Vt.) for a project at the Scripps Clinic Division of Orthopedic Surgery (San Diego). Force data from piezoelectric strips placed at strategic points in the knee assembly is collected by a wireless sensor chip and transmitted to a computer for analysis.

"Until now, no one has actually measured the loads inside the knee," said Steven Arms, CEO of Microstrain. Knee replacement designs have relied on mathematical models as a guideline for the demanding mechanical design behind artificial joint replacements. The real-time data from inside the knee will now be able either to confirm those models or provide more realistic re-visions, Arms said.

The Scripps Clinic has been working with MicroStrain Inc. and two other manufacturers since 1997 to design the implant and test it for use in clinical trials.

A novel knee joint design was required in order to include the sensor system, and packaging was a major challenge, said Arms. The sensor had to be totally sealed inside a titanium housing and guaranteed against leakage. The system itself had to be mechanically robust enough to withstand millions of knee movements.

The top half of the artificial knee is a conventional design with a polyethylene surface. The lower half of the joint is a custom design created by implant manufacturer DePuy Johnson and Johnson, (Warsaw, Ind.). Made from titanium, the unit includes all of the electronics and is hermetically sealed using laser welding. Four metal posts contact the upper half of the artificial joint and transmit strain forces to piezoelectric transducers underneath them. The transducers sense the local strain in the titanium, and the wireless sensor sends the data to an external antenna. A coil is wrapped around the knee to provide communications with the built-in sensor.

Microstrain engineers are in the process of improving the sensing and transmitter systems to make them more compact. Currently the chip is powered by a magnetic induction system that requires a bulky external pack. One option would be to include rechargeable batteries that could be occasionally recharged via an external power source.

Microstrain is also developing piezoelectric-based power systems for the wireless sensors, and knee movement might provide a source of mechanical energy for that type of sensor, Arms said.

"We now have some patents on an RFID transmitter design that would be much simpler than the transceiver in this system," he said. The same systems could also be reworked for spine and hip applications. Currently, measurements on those types of replacement components are done radiographically, but that method does not provide as much information as a wireless implant would yield, he said.

Another contributor to the project was NK Biotechnical, an engineering firm in Minneapolis.

More than 400,000 knee replacements were performed last year. As the senior populations of developed nations increase, that number is certain to rise.