ABSTRACT
A wireless telemetry system is described for measurement of forces acting upon massive orthopaedic implants, in man, to determine the load distribution in the fixation during routine activities and how this changes over long periods. The system comprises: the permanently implanted instrumented prosthesis, modified to enclose strain gauges and electronics, a single inductive link for powering the implant and telemetering the data, a microcontroller-based signal processor, a UHF radio link, and lap-top PC for real time data logging. The subject wears a small battery-powered inductive energiser, induction coil and microcontroller during the measurements. The strain gauges and implanted instrumentation are hermetically sealed inside cavities within the prosthesis, and are connected via a feedthrough to a small implanted induction coil. This coil is located outside the body of the prosthesis to maximise the power coupling efficiency, and is encapsulated using silicone rubber. Four prostheses have so far been implanted and data recorded over 1.5-3 years for each to date. This paper will focus on the electrical aspects of the telemetry system. Extracts of the axial force, bending moment and axial torque data will also be presented for the first Mk2 instrumented distal femoral replacement, during walking.
