Wireless sensors measure 3D force and torque data in human knee replacement

November 8, 2006 Historically, knee implants have been designed using predictions based on theoretical data. Now, a new smart knee replacement can wirelessly transmit multi-axis torque and force information directly from patients to a computer. These advances greatly enhance the capabilities of the first smart knee implant in 2004 that reported only knee compressive forces. The second generation implant provides a wealth of new information: twisting, bending, compressive, and shearing loads across the human knee - all reported dynamically and wirelessly. The data generated from this device will provide key inputs for new designs, techniques for implantation, and actual use of knee replacements. In-depth analysis can now be undertaken 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 improve design, refine surgical instrumentation, guide post-operative physical therapy and potentially detect the individual activities that would overload the implant.

Telemetry has been previously used to measure forces in the hip, spine and femur but the lack of available space in a knee replacement has prevented such a feat before. MicroStrain develops wireless microsensors for a wide variety of applications and has focused on making very small wireless strain sensing systems. The micro-miniature, micro-power nature of their wireless transmitter electronics and multi-channel strain measurement technology has enabled this breakthough. Batteries are completely eliminated by using a miniature coil within the implant, to harvest energy from an externally applied alternating field, which powers the implant. The remote powering coil is located on 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.

“MicroStrain is excited to contribute its wireless measurement technology to the team that made this breakthrough possible”, said Steven Arms, President of MicroStrain Inc. “Our expertise in multi-channel strain sensing, power management, hermetic packaging, and digital telemetry have allowed the realization of this revolutionary new smart total knee replacement”.

This project is an initiative undertaken by clinicians, scientists and industry beginning in 1993. Each participant contributed established expertise. Scripps Clinic biomechanical laboratory, under the direction of Darryl D'Lima, M.D and Clifford Colwell, M.D., has been utilizing the prototype of the replacement knee to perform evaluation implants for the past ten years. The clinical staff of the Scripps Clinic worked in tandem with MicroStrain Inc. and two other implant manufacturing companies to design and pretest these implants, making them ready for implantation in patients. After exhaustive safety testing, the implant was approved by Scripps Hospital's Internal Review Board for research purposes and is not yet available commercially.

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 stem portion is hollow - and this hollow space is used to house MicroStrain's 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 within 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 (6). The strain gauged knee was pre-calibrated prior to implantation. For the first time, orthopaedic researchers now have the tools to measure 3-D forces and torques in live human knees. As the recipient of this smart implant progresses during rehabilitation, 3-D load and torque data will be collected, for the first time, by Dr. D'Lima and his staff at Scripps Clinic during activities of daily living, including walking, climbing stairs, running, etc.

This system is currently a research device only and is not commercially available.

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