Sensor-based measurement for advanced monitoring and early detection of PE wear in total knee arthroplasties

Polyethylene (PE) inlays of knee endoprostheses are exposed to constant mechanical stress causing particle abrasion and volumetric wear. With the current inlay surveillance strategy, significant change often can be only detected when the implant has already failed. To reduce medical complications arising from implant malposition or failure, early and accurate assessment of the implant condition is of great value. We present a novel concept to monitor PE inlays based on an implanted sensor. Requirements of sensor integration into endoprostheses were discussed and specified for an inlay monitoring concept. A planar eddy current sensor for measuring the gradual descent of the femoral component was proposed where inductive and capacitive sensor elements form a resonant circuit. The potential sensitivity of the proposed measuring method was tested in an experimental setup using an RFID tag as a sensor substitute. A measurement concept, including the sensor and an extracorporal readout coil, was described and the validity was verified using finite element method (FEM) simulation of a simplified knee model. The experiments showed that a significant resonant frequency shift occurs in the sensor with decreasing distance to the femoral component. FEM simulation demonstrated that the sensor could be powered and readout extracorporeally through inductive coupling with an external readout coil. The proposed concept is a promising solution for feasible and accurate reading of the implant status designed to meet medical requirements. It can enable autonomous and routine monitoring as well as early detection of critical inlay deformation with a home-use device.