Measuring Fracture Healing with Smart Orthopaedic Implants

Abstract

There is a lack of methods for quantitatively assessing fracture healing. Current practice relies on a subjective mix of observation and indistinct imaging studies, yielding uncertainty and variability in fracture assessment, resulting in conservative clinical management, lost productivity, and increased costs which are exacerbated by delayed union and non-union cases. Previous researchers have shown that fracture union can be tracked by measuring changing strain for known applied loading on fixation plates as healing progresses. However, no devices exist for clinical use. This thesis aims to advance fracture healing assessment using strain-based measurements. Specific areas contributing to a functional implantable device include strain measurement technology, wireless power and communications, and a physical implant construction consistent with obtaining regulatory approval. A proposed solution suggests utilizing piezoresistive strain gauges in combination with a new wireless power technique: conductive transcutaneous energy transfer (CTET). A dual pathway is suggested due to an initial lack of knowledge of the optimal solution, which explores embedding this technology within 1/3 tubular plates as well as cortical lag screws. The electrical power, voltage delivery, safety, and electromechanical design requirements are assessed via an experimentally validated in-silico modelling approach showing 3.9mW power delivery to the smart screw and 17.5mW to the smart plate, providing evidence that these devices can be powered. The strain outputs and sensor requirements are assessed via an in-silico approach, with finite element modelling showing as much as 19μƐ divergence on the plate and 25μƐ on the screw between healing states. The results are confirmed experimentally in cadaver studies showing 22μƐ of strain discrepancy on the plate between fracture states. The experimental data resulted in a final prototype for an instrumented lateral malleolar smart plate implant powered by CTET that communicates wirelessly with an inductive phase shift keying mechanism. The design was constructed and tested in a benchtop saline leg phantom, proving feasibility by predicting fracture healing in a wirelessly powered, full-metallic bodied lateral malleolar smart plate implant, that incorporated miniaturized electronics exhibiting a preliminary 13mm x 6mm x 3.5mm flexible PCB footprint with capacity for further miniaturization and a design consistent with gaining regulatory approval.