IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 1, JANUARY 2015 4002303 In Vivo Monitoring of Orthopaedic Implant Wear Using Amorphous Ribbons David Okhiria 1 , Dia E. Giebaly 2 , Turgut Meydan 1 , Samuel Bigot 3 , and Peter Theobald 4 1 Wolfson Centre for Magnetics, Cardiff School of Engineering, Cardiff CF24 3AA, U.K. 2 Department of Trauma and Orthopaedics, University College Hospital, London NW1 2BU, U.K. 3 High Value Manufacturing Group, Cardiff School of Engineering, Cardiff CF24 3AA, U.K. 4 Biomedical Engineering Research Group, Cardiff School of Engineering, Cardiff CF24 3AA, U.K. The wearing process of an ultrahigh molecular weight polyethylene (UHMWPE) bearing surface in orthopaedic surgical implants cannot be tracked at present; hence, implant failure only becomes apparent when the patient experiences some discomfort. In exploring the integration of wireless stress sensors into UHMWPE, this paper describes the first step in achieving in vivo monitoring of implant wear using magnetostrictive amorphous ribbons. Index Terms— Amorphous materials, biomedical applications, magnetic sensors, orthopaedic implants. I. I NTRODUCTION J OINT replacement surgery (or joint arthroplasty) is now an established surgical procedure, with ∼170 000 hip or knee replacements performed annually with the U.K. [1]. Joint arthroplasty is a complex surgical procedure, requiring precise alignment of the new and existing bearing surfaces, combined with careful re-balancing of the supporting connective tissues. That cases often exceed 15-years longevity [2] is due to a surgical skill, evolving component designs, and the use of enhanced bearing materials that minimize the wearing of these critical surfaces. Polyethylene (PE) bearings are the most common, having evolved through ultraheight molecular weight, highly cross linked, and now chemically impregnated derivatives. Simulator and computational studies have demonstrated the superior tribological characteristics of these materials over extended time periods; however, such performance is only achieved within an optimal biomechanical environment [3]. The realities and complexities of orthopaedic surgery, however, means a proportion of all PE bearings experience stresses that fall outside of the optimal boundaries, typically as a consequence of minor implant malpositioning and/or soft-tissue imbalance. Such instances are likely to cause accelerated wear of the bearing surfaces and/or fracture, leading to premature failure [3]. The integrity of the bearing surfaces is critical to implant longevity. Ongoing monitoring is, however, problematic due to PE being radiolucent (i.e., invisible when viewed via X-ray), meaning that cases of an accelerated implant wear may only be detected when the patient reports discomfort and/or pain. Such pain is likely to be caused by the implant loosening, a severe and irreversible physiological response to foreign debris. Indeed, implant loosening is the primary cause of premature implant failure [4]. Such instances require further Manuscript received June 13, 2014; revised September 2, 2014; accepted September 4, 2014. Date of current version January 26, 2015. Corresponding author: T. Meydan (e-mail: meydan@cardiff.ac.uk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2356715 surgery, with an increased morbidity and mortality risk, and significant financial cost. Early detection of high wear-rate cases would allow for proactive treatment (physiotherapy, orthotics, and so on), providing the opportunity to avert premature implant failure. International research groups have, for a number of years, been developing technologies to acquire highly valuable in vivo biomechanical data, and have achieved success across very small cohorts. Their techniques, however, typically require extensive retrofitting of strain gauges, batteries, and antennae, and so is not a viable solution for mass production [5], [6]. One way around this is the utilization of the magnetomechan- ical (Villari) effect exhibited by amorphous metallic glasses, in which their magnetic properties vary as a function of mechanical strain. Magnetoelastic anisotropy is introduced in the amorphous material as a result of the strain, and this produces changes in its magnetic permeability. This effect has been researched extensively and proposed as a basis for viable sensors. This paper integrates amorphous magnetostrictive ribbons into ultrahigh-molecular weight PE (UHMWPE), evaluating whether the proportional change in their magnetic properties when subjected to a mechanical stress offers the potential to ultimately enable a mass produced instrumented orthopaedic implant that may prevent premature failure. II. THEORY Amorphous ferromagnetic materials have proven to be quite indispensable in a variety of applications [7], [8]. In magnetics, they are non-crystalline alloys of iron, cobalt, or nickel and metalloids (predominantly boron and silicon), which are produced by melt spinning and rapid quenching techniques and are usually produced in ribbon and wire forms. Ferromagnetic materials usually consist of domains, which are individual randomly magnetized regions, such that the resultant magnetization within the structure is nearly zero. These domains define a relationship between the physical and macroscopic material properties [9], and a thorough analysis of their characteristics has led to effective harnessing such as in stress sensors. When an external mechanical stress is applied 0018-9464 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.