Efficacy of bone stimulators in large-animal models and humans may be limited by weak electric fields reaching fracture

Abstract

Noninvasive electronic bone growth stimulators (EBGSs) have been in clinical use for decades. However, systematic reviews show inconsistent and limited clinical efficacy. Further, noninvasive EBGS studies in small animals, where the stimulation electrode is closer to the fracture site, have shown promising efficacy, which has not translated to large animals or humans. We propose that this is due to the weaker electric fields reaching the fracture site when scaling from small animals to large animals and humans. To address this gap, we measured the electric field strength reaching the bone during noninvasive EBGS therapy in human and sheep cadaver legs and in finite element method (FEM) models of human and sheep legs. During application of 1100 V/m with an external EBGS, only 21 V/m reached the fracture site in humans. Substantially weaker electric fields reached the fracture site during the later stages of healing and at increased bone depths. To augment the electric field strength reaching the fracture site during noninvasive EBGS therapy, we introduced the Injectrode, an injectable electrode that spans the distance between the bone and subcutaneous tissue. Our study lays the groundwork to improve the efficacy of noninvasive EBGSs by increasing the electric field strength reaching the fracture site.

Computational modeling of dorsal root ganglion stimulation using an Injectrode

“…The Injectrode is composed of a platinum/iridium micro coil, which is designed to be introduced through a needle (18g, 1.27mm) and positioned around a specific neuroanatomical target. Once in place, it assumes a highly conforming and flexible structure, creating a clinical-grade electrode platform [26,43,53]. This unique design is intended to allow the Injectrode to conform around the target neuroanatomy, enabling access to challenging anatomical sites, including the DRG.…”

“…This improvement can be attributed to the impedance characteristics of the pathways between the active Injectrode, the “return electrode,” and the impedance between two TES patches overlaying the two collectors. Experimental results indicate that the deep penetration of current in a monopolar configuration (single Injectrode) is approximately 5%, whereas in a bipolar configuration (two Injectrodes), it increases to 20% [43,53].…”

“…The Injectrode system employs TES electrodes that wirelessly transfer charge to subcutaneous collectors, which is the uncoated wire section of an Injectrode placed under the skin during the final step of the injection procedure. This non-invasive stimulation setup minimizes associated risks and complications inherently part of percutaneous electrode systems [43,[53][54]. To investigate the design parameters of the Injectrode for this type of bipolar stimulation, we developed a FEM model of a noninvasive transcutaneous charge delivery system in a full-scale human body model with Injectrodes placed bilaterally at the L5 DRG.…”

“…The delivered electrode is therefore larger in diameter than the needle from which it was deployed, thereby reducing the chances of migration. Once the Injectrode is delivered to the target structure, then the insulated lead is extruded back to the superficial tissue layers where the subcutaneous charge collector is placed [43,53]. Electrical current is delivered transcutaneously to the charge collector located directly beneath the skin surface using noninvasive skin adhesive patch electrodes, eliminating the need for implantable pulse generators or wires perforating the skin.…”

Computational modeling of dorsal root ganglion stimulation using an Injectrode

“…The delivered electrode is therefore larger in diameter than the needle from which it was deployed, thereby reducing the chances of migration. Once the Injectrode is delivered to the target structure, then the insulated lead is extruded back to the superficial tissue layers where the subcutaneous charge collector is placed [26,27]. Electrical current is delivered transcutaneously to the charge collector located directly beneath the skin surface using noninvasive skin adhesive patch electrodes, eliminating the need for IPGs or wires perforating the skin.…”