A novel flexible capacitive load sensor for use in a mobile unicompartmental knee replacement bearing: an in vitro proof of concept study

Instrumented knee replacements can provide in vivo data quantifying physiological loads acting on the knee. To date instrumented mobile unicompartmental knee replacements (UKR) have not been realised. Ideally instrumentation would be embedded within the polyethylene bearing. This study investigated the feasibility of an embedded flexible capacitive load sensor. A novel flexible capacitive load sensor was developed which could be incorporated into standard manufacturing of compression moulded polyethylene bearings. Dynamic experiments were performed to determine the characteristics of the sensor on a uniaxial servo-hydraulic material testing machine. The instrumented bearing was measured at sinusoidal frequencies between 0.1 and 10 Hz, allowing for measurement of typical gait load magnitudes and frequencies. These correspond to frequencies of interest in physiological loading. The loads that were applied were a static load of 390 N, corresponding to an equivalent body weight load for UKR, and a dynamic load of ±293 N . The frequency transfer response of the sensor suggests a low pass filter response with a -3dB frequency of 10 Hz.The proposed embedded capacitive load sensor was shown to be applicable for measuring in vivo loads within a polyethylene mobile UKR bearing.