Investigations into the ability of Piezoelectric Sensors to Monitor the Integrity of the Cemented Bond between Bone and Implant

Abstract

This study investigates the utility of piezoelectric transduction to assess the structural health of a system through impedance analysis, with application to the field of ‘smart’ orthopaedic implants. The work is motivated by the high proportion of orthopaedic implant failures that occur due to loosening of the bond between the implant and the corresponding bone surface. The ultimate aim is to prove that piezoelectric sensors embedded within orthopaedic implants have the potential identify implant loosing before it would be shown in imaging techniques.

Orthopaedic knee implants were selected as a case study for proof of concept for the proposed health monitoring system. Three distinct experiments were conducted: 1) Small piezoelectric sensors are attached to model tibial trays which are in turn attached with bone cement to sawbone blocks. The measured sensor impedance over a range of input frequencies is measured and analysis of the frequency impedance traces is carried out to determine what changes in the trace are indicative of the bone cement between the sawbone and aluminium curing; 2) Commercially available tibial trays cemented to sawbone tibias are progressively loosened under a fatigue load in a compressive testing rig. Results from three Linear Variable Differential Transducers (LVDTs) measuring the micromotion between the implant and sawbone are compared with frequencyMimpedance traces taken from a piezoelectric sensor attached to the top side of the tibial tray; and 3) Varying amounts of bone cement is used to cover the surface between a sawbone block and model tibial tray. Frequency-impedance readings are taken from a piezoelectric sensor adhered to the top side of the tray. Support vector machines are used to classify between the varying amounts of cement on each test sample.

Experimental results and data analysis demonstrate the potential of piezoelectric sensors ability to provide information on the integrity of bone cement bond. Findings include: 1) Piezoelectric sensors can determine at what point bone cement bond between sawbone and an aluminium plate has cured; and 2) It is possible to identify different levels of cement coverage between sawbone and aluminium plate with an accuracy of up to 92 % with piezoelectric sensors. These findings establish the veracity of piezoelectric transduction as a means of identifying orthopaedic implant loosening in vivo. This investigation provides a firm basis for future work bringing the ideal of using piezoelectric sensors as a technique for detecting loose implants in vivo closer to becoming a reality.