Implant-Based UHF RFID-Enabled Sensor to Detect Loosening of Knee Prosthesis

This paper presents a novel long-term electronic implant enabled by passive UHF RFID technology to detect early signs of loosening between orthopedic knee prostheses and bones. It is composed of an AMS SL900A chip, connected to a miniaturized antenna matched to a bio-tissue environment. The implant embeds an external capacitive membrane sensor to detect force variation correlated with a loss of adhesion between the tibia component of the total knee prosthesis and the prosthesis. This stainless steel membrane sensor features high endurance and can detect the first signs of loosening from an applied compression force of 1.25 N. It corresponds to a measured capacitance change of 10fF. The overall size of the RFID implant inserted in the tibial tray is 12.65mm x 12.3mm with a thickness of 2.63 mm, including the casing. Both simulations and practical measurements validate this novel work for various simulated bio-tissue environments in the presence of metal. Besides, a theoretical study on SAR exposure confirms the compatibility with regulatory constraints. Finally, a test bench composed of a force bench, an external RFID reader, and the RFID implant inserted within the tibial component of a knee prosthesis provides proof of concept for the proposed system.