This paper describes an in-vivo polyethylene wear and knee prosthesis longevity estimation approach using an implantable ligament imbalance metric system. Four piezoelectric elements are used to measure the axial force components, on the tibial tray surface and their center of pressure is calculated. This can be used, along with a knee simulator, to estimate polyethylene wear, and the state of the implanted knee prosthesis. Surgeons can exploit this to improve the post operative rehabilitation measures, whereas prosthesis manufacturers can use it to improve prosthesis’s strength, geometry and life. The proposed selfpowered measurement and telemetry system does not require any external power.