Powering Smart Orthopedic Implants Through Near-Field Resonant Inductive Coupling

Our research aims to enhance smart orthopedic knee implants used in Total Knee Arthroplasty (TKA). With the projected quadrupling of TKA demand by 2030 due to factors like aging populations, rising obesity rates, and broader indications for younger patients, our focus is on instrumented medical implants to measure knee parameters. In this paper, we report the optimization of a wireless power transmission system for powering smart knee implants, employing an established HF Near-field Resonant Inductive Coupling (NRIC) technique at $13.56 \,\mathrm{M}\mathrm{Hz}$ inside the stem of a tibial knee implant. We propose a pragmatic optimization approach in this study, guided by the integration constraints of a knee implant and validated by orthopedic surgeons through cadaveric specimen testing. Finite Element simulations guided the selection of a frontal 3-turn solenoid (called “paperclip” coil) at the Rx side, located at the tip of the stem, which demonstrated balanced performance metrics and reasonable volume occupancy (1.6 cm ³ ). Power transfer measurements were conducted through conductive solutions mimicking skin, muscle, and bones. At $13.56 \,\mathrm{M}\mathrm{Hz}$ , a power transfer efficiency $\eta$ of 30% and 7.5% ( $300 \,\mathrm{m}\mathrm{W}$ and $75 \,\mathrm{m}\mathrm{W}$ at $1 \,\mathrm{W}$ input power) was achieved at Tx-Rx distances of $25 \,\mathrm{m}\mathrm{m}$ and $40 \,\mathrm{m}\mathrm{m}$ respectively. The proposed solution was implanted in a cadaveric specimen : $250 \,\mathrm{m}\mathrm{W}$ was obtained at an estimated $30 \,\mathrm{m}\mathrm{m}$ distance for an input power of $1 \,\mathrm{W}$ at the Tx side. For the same distance, we also performed a successful DC power provision up to $64 \,\mathrm{m}\mathrm{W}$ at $3 \,\mathrm{V}$ DC and data transfer functions at $26\, \mathrm{kbit\,s}^{-1}$ in the cadaver. The proposed system, with its integration strategy, holds promise in powering advanced sensor functions, contributing to the identification and monitoring of postoperative complications and potentially reducing the need for long-term revisions.