An instrumented prosthesis for knee joint force measurement in vivo

Although forces acting across the hip joint are now well documented both by analysis and in vivo measurement, those acting at the knee have so far only been determined analytically using a combination of mathematical models, kinematic data and ground reaction forces. These methods are subject to many simplifications and assumptions. The aim of the present work is to measure the forces acting across the femero-tibial joint of a permanently implanted total knee replacement in vivo. The tibial tray is designed as a double 6 degree-of-freedom loadcell (for separate measurement of the force system acting on the medial and lateral compartments), instrumented internally with strain gauges and integral electronics. The forces and moments to be measured on each compartment are the axial force and torque, the antero-posterior and medio-lateral shear forces, and the X and Y locations of the centre of applied load on the tibial tray. A total of 24 strain gauges are arranged around the periphery of the loaded member of each compartment (loaded by the femoral component via a plastic bearing surface), wired as 12 half bridges. Developed strains are amplified, digitised and telemetered at 100 samples per second per channel (total of 24 channels). Since the strain gauges are sensitive to all the measured variables, analysis will involve an iterative matrix method or nonlinear techniques. A calibration rig is used to determine the response of the gauges to each applied force and moment. Power is supplied and data telemetered via an inductive link between a small implanted coil and one mounted around the tibia during measurements. A finite element model was used to determine the optimum gauge locations for best sensitivity to the applied forces, as well as to check for adequately low stress levels.