Sensing and Energy Harvesting Performance, and Fatigue Life of Embedded Piezoelectric Transducer in Total Knee Arthroplasty
Authors: Safaei Mohsen , Anton Steven R. 
Published: November 2016

Growing demands for Total Knee Arthroplasty (TKA) and also total knee revision surgery combined with the cost, risk, and complication of the surgery have led to numerus attempts to improve surgical techniques, implant design, and postoperative orthopedic therapies. Although abundant information about knee function and reaction forces and moments have been provided by researchers through biomechanical models, cadaver testing, in-vitro testing, and limited in-vivo measurements, rigorous real-time in-vivo data from knee implants is still required to improve the performance of TKAs. Instrumented knee implants using piezoelectric (PZT) transducers have promising potential to satisfy clinical needs in terms of continuous in-vivo data acquisition, self-powered operation, and retention of prevalent implant design, which can ultimately lead to improved patient satisfaction. In this study, a simplified Ultra High Molecular Weight (UHMW) Polyethylene TKA bearing geometry with an embedded PZT on the bottom surface is proposed and investigated to analyze sensing and power harvesting, and longevity of the conceptual design. As a result, this work is separated into two distinct sections. The first part provides an evaluation study on the performance of the design in terms of output voltage and power using both simulations and experimental tests. Finite element analysis (FEA) is employed to model the stress-strain behavior of the system and to develop effective force reaction on the PZT transducer. An analytical model is used to describe the electromechanical behavior of the PZT transducer under the effective force predicted by FEA, and the output voltage and power of the system are simulated. Furthermore, results obtained from modeling are validated through experimental compression testing using simulated gait conditions. Embedding a PZT element in the knee bearing may cause changes in stress distribution in UHMW and as a result the variation in the fatigue life of the bearings with encapsulated PZTs is considered as a remarkable factor to investigate. Therefore, in the second part of the work, a parametric study on the effect of dimensional parameters on the longevity and electromechanical performance of the design is performed. High cycle stress life of the polyethylene component with embedded PZT transducer as well as transferred force to the PZT and generated voltage under periodic knee load are studied. . The diameter and depth of the pocket machined in the UHMW bearing, the thickness ratio of the PZT element to the UHMW component, and modification of the contact edges inside the PZT pocket and PZT are considered as effective geometrical parameters on the fatigue life of the UHMW bearing and are studied individually. Two designs are investigated; the initial design with sharp corners and a revised design with filleted corners. The results show a significant fatigue life improvement by adding a fillet radius modification on the sharp corners of the UHMW and PZT components accompanied by a slight reduction in output voltage. The effect of pocket diameter is dependent on the geometry and for the initial design the fatigue life and output voltage increase when diameter increases. For the revised design, fatigue life decreases for large fillet radii and increases for small fillet radii and converge as diameter is increased, whereas the output voltage slightly increases with large pocket diameters. Pocket depth has a significant reverse effect on fatigue life and output voltage of the PZT, such that a 0.05 mm deeper pocket results in no force transfer and no voltage but improved fatigue life.