Capacitive stimulation-sensing system for instrumented bone implants: Finite element model to predict the electric stimuli delivered to the interface

Background: Prevalence of orthopaedic replacements are increasing around the world. The main cause of revision remains associated to the interface loosening. In this work, a computational study using the Finite element method was developed to predict the electric field stimuli delivered to trabecular bone structures, as well as to predict the sensing ability to detect different bone-implant interface scenarios.

Methods: Three finite element models were developed: two simplified models, including a Gyroid TMP structure, and a realistic model based on microCT scan of a trabecular bone from sheep vertebra. Simulations were performed using a co-surface capacitive technology for stimulating and sensing bone-implant interfaces. Different fixation scenarios were considered, namely by establishing bone-stimulator gap sizes up to 1 mm (from fixation to massive loosening scenario). Electrodes were excited with sinusoidal and square electric signals up to 10V voltage and 64kHz frequency.

Results: Simplification of bone geometry resulted in significant electric stimuli differences compared to the realistic bone geometry. Realistic modelling allowed to observe that, in the fixation scenario, the electric field stimuli decreased 85% from the sensor interface to a parallel plane 2 mm apart from such interface. A significant influence of the bone-stimulator distance on the electric stimuli was found: the electric stimuli magnitudes varied in the range between 0.38 V/mm (fixation scenario) and 4.8 mV/mm (massive loosening scenario) for voltages up to 10V. Strong frequency-dependent behaviours were also observed in the electric stimuli: their magnitudes can reach 106-fold decreases when the excitation frequency is decreased from 32 kHz to 14 Hz CONCLUSION: This study points out the inability of our two simplified models to predict the electric stimulation provided to different bone-implant interface scenarios. Results highlight that co-surface stimulators can deliver osteogenic electric stimuli along trabecular bone structures, ensuring low electric power excitations. Moreover, realistic models strongly enhance the sensing predictability of the bone-implant fixation states. These new and significant evidences provide a strong support to integrate co-surface capacitive into bioelectronic implants for both therapeutic and sensing operations.