This interesting paper (click for the PDF version, available online), published in 1999 by Townsend et al., presents an instrumented tibial baseplate for in-vivo measurements of knee tibiofemoral forces. A multichannel telemetry system is combined with strain gauges; the whole system is alimented wirelessly by means of magnetic induction.
Basically, electrical energy is transferred from a primary external coil, properly positioned around the patient’s leg during the medical check, to a secondary coil, smaller and embedded in the tibial tray stem. Approximately 40 mW of power are transferred and supply the whole telemetry system, which can transmit data to an external receiver within a range of 3-5 meters.
Townsend, C.P., and Arms, S.W., Hamel, M.J., ”Remotely Powered, Multichannel, Microprocessor Based Telemetry Systems for a Smart Implantable Total Knee Implant”, SPIE’s 6th Annual Int’l conference, Date: Mar. 1999.