Integration of a wireless lock-in measurement of hip prosthesis vibrations for loosening detection

A new solution for the integration of a wireless measurement system intended for the detection of hip prosthesis loosening by vibration analysis is presented. Small vibration amplitudes in the presence of noisy sensor signals of the excited prosthesis are measured with a digital lock-in amplifier which is integrated in the prosthesis together with a controller and a passive telemetry working at 125 kHz. The integrated controller receives the necessary reference signal information to synchronize the lock-in amplifier from the external reader station via RF field. Measurements were taken at a bare femur and an artificial thigh where an electrodynamic audio shaker was fixed at or near the inner femur condyle. In order to explain and to predict the natural frequencies a problem oriented one-dimensional mechanical network model was developed.