Artificial Knee Wirelessly Communicates

Verifying theoretical calculations in real-world medical situations has always been a tricky proposition, especially in space-restricted locations such as replacement joints. Recent results from a research collaboration of clinicians, scientists and the industry initiated in 1993 indicate a breakthrough in this area. The researchers developed and implanted a full artificial knee replacement that wirelessly communicates 3-D torque and force data for external computer analysis. MicroStrain developed the sensing and communication technology including a batteryless means of powering the circuitry.

An array of 12 high-sensitivity piezoresistive strain gauges embedded within the implant makes the torque and force measurements. Prior to implantation, the strain-gauge instrumented knee was pre-calibrated. To get the information outside of the body, a micro-miniature, micro-power wireless transmitter sends digital data to an external analytical instrument.

An energy harvesting technique powers the implant that uses a miniature coil and an externally applied alternating field, eliminating the need for batteries. Mounted to the outside of the patient's shin, the remote powering coil is located away from the knee. Data from the strain gauge measurements are converted into 3-D torques and forces about the knee by the external computer using a stored calibration matrix.

The wireless sensing system measures the twisting, bending, compressive and shearing loads across the implanted knee. Based on analyzing the forces and torques transmitted across the knee joint during normal human activities - such as stair climbing, rising from a chair and walking - researchers will have information for design improvements, to refine surgical instrumentation and to guide post-operative physical therapy. The measurements may also detect activities that would overload the implant.

Get more information on MicroStrain's wireless strain gauge electronics.