A Coaxial Dipole Antenna for Passively Sensing Object Displacement and Deflection for Orthopaedic Applications

A promising approach for monitoring and predicting the course of bone fracture healing is by measuring the mechanical load-sharing between the healing callus and the implanted fixation hardware. Previous technologies have used implantable sensors which require modification to the fixation hardware and may carry long term biocompatibility risks. The objective of this paper was to optimize and evaluate a method of externally sensing hardware load-sharing based on the electromagnetic near field effects of a radio-frequency antenna. A series of parametric experiments was conducted to optimize the dimensional parameters of a coaxial dipole antenna to improve the antenna’s sensitivity to displacement of a metal plate. The results of the parametric tests guided the design of an optimized antenna, including a coiled loop antenna structure. The antenna was then evaluated for its efficacy in sensing the displacement of a metal plate as well as the deflection of an orthopaedic fracture fixation plate due to an applied load via physical experiments and computational simulations. The antenna’s resonant frequency was sensitive to the displacement of a metal plate, and followed an inverse-square relationship with plate distance. The antenna was also able to sense the bending deflection of the mechanically loaded fracture plate, with the resonant frequency following an approximately linear relationship with applied load. Computational finite-element electromagnetic predictions closely matched the experimental data. This method of sensing plate deflections may be effective for measuring the mechanical load sharing in fractured bones in order to monitor and predict the course of fracture healing.