Experimental Evaluation of Sensing and Energy Harvesting Behavior of Implanted Piezoelectric Transducers in Total Knee Replacement

Total knee replacement has been utilized to restore the functionality of diseased knee joints for more than four decades. Today, despite the relatively high level of patient satisfaction, still about 20% of patients are not fulfilled with their surgical outcomes in terms of function and reduction in pain. There is still an ongoing discussion on correlating the postoperative functionality of the joint to intraoperative alignment, which suffers from lack of in vivo data from the knee after surgery. However, it is necessary to mention that using computer assisted surgical techniques, the outcomes of knee replacement procedures have been remarkably improved. In order to obtain information about the knee function after the operation, the design of a self-powered instrumented knee implant is proposed in this study. The design is a total knee replacement ultra high molecular weight polyethylene insert equipped with four piezoelectric transducers distributed in the medial and lateral compartments of the bearing. The piezoelectric elements are employed to measure the axial force applied on the tibial insert through the femoral component of the joint as well as to track the movement in the center of pressure. In addition, generated voltage from the piezoelectrics is harvested and stored to power embedded electronics for further signal conditioning and data transmitting purposes. The performance of the instrumented implant is investigated via experimental testing on a fabricated prototype in terms of sensing and power harvesting capacity. Piezoelectric force and center of pressure measurements are compared to the actual quantities recorded from the load frame and pressure sensitive films in order to evaluate the performance of the sensing system. The output voltage of the piezoelectric transducers is rectified and stored in a capacitor to evaluate the energy harvesting ability of the system. The results show only a small level of error in sensing the force and the location of center of pressure. Additionally, a 4.9 V constant voltage is stored in a 3.3 mF capacitor after 3333 loading cycles. The sensing and energy harvesting results present the promising potential of this system to be used as an integrated self-powered instrumented knee implant.