Smart implant: the biomechanical testing of instrumented intramedullary nails during simulated callus healing using telemetry for fracture healing monitoring

Abstract

The purpose of this study is to investigate the effect of loading during long bone fracture healing in-vitro and in-vivo. Fracture healing has until now only been monitored using radiographs and ultrasound. An intramedullary nail instrumented with strain gauges has the potential to monitor loading in-vivo during bone fracture healing. Strain has been previously monitored over time though external fixation devices however there has been no published data about monitoring through a nail. The load carried by a telemetric intramedullary nail during simulated fracture healing is monitored in-vitro with the aid of custom designed jigs, integrated in a biomechanical test frame. Clinically predetermined loading conditions are applied to the construct and synthetic bone composites are used developed to simulate mechanical strength of early to mature osteogenic bone, approximating natural healing processes. Four different synthetic bone composites have been designed and developed to mimic the mechanical properties of granulation tissue, fibrous tissue, cartilaginous tissue and immature bone. Three different generations (GEN I – IIIa) of intramedullary nails were developed and biomechanically tested in-vitro. GEN I and II were purely biomechanical nails that underwent compression, torsional and 4pt bend tests. Different fracture patterns and callus morphology were simulated and tested biomechanically. Circumferential and segmental application of the synthetic materials were applied on the artificial fractured bone instrumented with GEN I. Observations from live animal study provided x-rays from which callus growth patterns were extracted and repeated in-vitro. Cadaveric biomechanical tests and pre-clinical trial of GEN IIIa was conducted. The aim was to repeat the biomechanical tests while at the same time monitoring healing with an instrumented nail implanted in an induced fractured, ovine left hind limb. A loading rig was designed for the in-vivo test. The hypothesis proposed is that forces experienced by an intramedullary nail will progressively decrease as fracture heals. Results from GEN I have shown that strain measurement can be monitored during fracture healing in-vitro. The GEN IIIa nail is yet to be tested in-vivo for the same biomechanical tests for comparison. There is currently no published study on simulating fracture healing with accuracy.