Wireless Sensors Measure Data in Knee Implants

Orthopedic implants keep getting smarter. For the first time, a patient has received an investigational, full artificial knee replacement that can wirelessly report back to computers -- digitally, and in three-dimensions -- torque and force data. These advances greatly enhance the capabilities of the first smart knee implant, in 2004, which reports only compressive forces in the knee, said their manufacturer, MicroStrain Inc.

MicroStrain wireless sensors (inset, right) measure 3-D force and torque data in live human knee replacement.

The knee-sensor project is an initiative undertaken by clinicians, scientists and industry beginning in 1993. Scripps Clinic biomechanical laboratory in California, under the direction of physicians Darryl D’Lima and Clifford Colwell, has been using the prototype replacement knee to perform evaluation implants for 10 years. The clinical staff of the Scripps Clinic worked in tandem with MicroStrain, based in Williston, Vt., and two other implant manufacturing companies to design and pretest the implants, making them ready for use in patients. After exhaustive safety testing, the procedure was approved by Scripps Hospital's internal review board for research purposes.

This work has led to award-winning publications on in vivo knee compression forces -- measured by the first-generation smart total knee replacement. The second implant builds on this experience to advance the technology to the next step, MicroStrain said.

The second generation implant provides a wealth of new information -- twisting, bending, compressive and shearing loads across the human knee -- all reported dynamically in vivo. These data will provide key inputs for new designs, techniques for implantation and actual use of knee replacements. Historically, knee implants have been designed using predictions based on theoretical data. With the new technology, the smart total knee replacement can transmit multi-axis loading information directly from patients.

In-depth analyses can now be made of forces and torques transmitted across the knee joint during normal human activities, such as stair climbing, rising from a chair and walking. The results of this analysis can be used to develop design improvements, refine surgical instrumentation, guide postoperative physical therapy and potentially detect individual activities that would overload the implant.

Telemetry has been used to measure forces in the hip, spine and femur, but the available space in a knee replacement had previously posed severe barriers. MicroStrain, which develops wireless microsensors for a wide variety of applications, has focused on making very small wireless strain sensing systems. The second-generation implant handles 12 channels of strain data, versus only four strain channels in the older system.

The microminiature, micropower nature of the wireless transmitter electronics and multichannel strain measurement technology enabled the breakthough. Batteries are completely eliminated by using an integral miniature coil within the implant to harvest energy from an externally applied alternating field that powers it. The remote powering coil is secured to the outside of the patient’s shin, away from the knee.

Using a wireless antenna, the implant transmits digital sensor data to a computer in a readable format. The twelve strain gauges are input to a computer, which uses a stored calibration matrix to convert the raw strain data into 3-D torques and forces about the knee.

A custom titanium alloy total knee replacement was provided as the basis for the device. The tibial component accepts standard, commercially available high-molecular-weight polyethylene inserts. The hollow stem portion is used to house the wireless strain gauge electronics. A polyethylene cap is threaded onto the distal end of the stem and protects the hermetically sealed radio antenna. The electronics, including the sensing elements, are fully contained in the implant, which is hermetically sealed using laser welding techniques. The finished, sealed implant is tested for hermeticity using fine helium leak-detection methods -- the same methods that are used to test advanced pacemakers.

The array of twelve sensitive piezoresistive strain gauges were embedded within the implant’s custom designed tibial component. The strain gauged knee was precalibrated prior to implantation.

As the recipient of the smart implant progresses during rehabilitation, 3-D load and torque data will be collected by Dr. D’Lima and his staff at Scripps Clinic during regular activities including walking, climbing stairs and running. The system is a research device only, and is not yet available commercially.

For more information, visit: www.microstrain.com