Wireless implantable sensor for non-invasive, longitudinal quantification of axial strain across rodent long bone defects

Abstract

Bone development, maintenance, and regeneration are remarkably sensitive to mechanical cues.Consequently, mechanical stimulation has long been sought as a putative target to promote endogenous healing after fracture. Given the transient nature of bone repair, tissue-level mechanical cues evolve rapidly over time after injury and are challenging to measure non-invasively. The objective of this work was to develop and characterize an implantable strain sensor for non-invasive monitoring of axial strain across a rodent femoral defect during functional activity. Herein, we present the design, characterization, and in vivo demonstration of the device's capabilities for quantitatively interrogating physiological dynamic strains during bone regeneration. Ex vivo experimental characterization of the device showed that it exceeded the technical requirements for sensitivity, signal resolution, and electromechanical stability. The digital telemetry minimized power consumption, enabling long-term intermittent data collection. Devices were implanted in a rat 6 mm femoral segmental defect model and after three days, data were acquired wirelessly during ambulation and synchronized to corresponding radiographic videos, validating the ability of the sensor to non-invasively measure strain in real-time. Lastly, in vivo strain peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/142778 doi: bioRxiv preprint first posted online Page 2 of 24 measurements were utilized in a finite element model to estimate the strain distribution within the defect region. Together, these data indicate the sensor is a promising technology to quantify local tissue mechanics in a specimen specific manner, facilitating more detailed investigations into the role of the mechanical environment in dynamic skeletal healing and remodeling.

A Smart Implantable Bone Fixation Plate Providing Actuation and Load Monitoring for Orthopedic Fracture Healing

“…Piezoelectric sensors operate very differently than strain gauges, which have been more commonly used in similar load-monitoring applications. Because of this, the sensing pathway is significantly different than what is found in similar works [29]. work, though other authors prefer other quantities [82,83].…”

“…Instead, many have attached strain gauges to fixation plates to indirectly measure the stiffness of the fracture callus [27,28]. As the stiffness of the callus increases, the measured strain decreases (Figure 1 Strain gauge-based stiffness data can also be used to evaluate the effectiveness of novel treatment methods throughout the healing process [29,30]. Endpoint data, such as stiffness and vascularization measurements, are often supplemented with X-rays to obtain a better picture of the progress of healing over time.…”

“…The actuation performance of the fixation plate was measured using the Electroforce 3200 mechanical tester (TA Instruments, New Castle, DE, USA) mechanical tester. The fixation plate was attached to two 6061 aluminum blocks with a gap between them to simulate a fractured bone following previously established methods [29]. Rubber Right: Teflon gap (stiffness = 232 MPa) to simulate immature cortical bone.…”

“…The telemetry packaging described in this work uses the same materials and methods for the enclosure and wires to the fixation plate as those used in previous implants used in this animal model [29,81], except the 3D printed packaging is of a different size. While 3D…”

“…printed packaging and the polymer coating are limited to short-term implants (< 29 days [88]), it is commonly used in animal studies like this work [29,81].…”