Surgically-induced deformation in biodegradable orthopaedic implant devices

The biocompatibility and mechanical performance of biodegradable metals (e.g. magnesium, iron, and zinc-based alloys) in orthopaedic-targeted applications are contingent on limiting the rate of corrosion in vivo throughout the bone healing. Concurrently, the surgical procedure for the implantation of internal bone fixation devices may impart plastic deformation to the device, potentially altering the corrosion rate of the device. However, the potential effect of the surgical implantation procedure on the mechanochemical performance of metallic degradable orthopaedic devices in vivo remains largely unresolved. The objective of the present study is to develop a robust technique that permits the quantification of the strain introduced due to surgical implantation of degradable orthopaedic devices. Specifically, a novel combined experimental-modelling approach based on 3D laser scanning in situ and the finite element method is utilised to quantify the plastic strain introduced to a bone fixation plate following surgical implantation in a cadaveric porcine model where the plate is based on a ternary magnesium-zinc-calcium alloy (ZX10). The magnitude of plastic strains determined by the above approach for the Mg craniofacial miniplate confirms that the surgical procedure itself has the potential to enhance the corrosion rate of the Mg alloy in an accelerated and potentially localised manner. STATEMENT OF SIGNIFICANCE: Biodegradable metallic orthopaedic implant devices have emerged as a potential alternative to permanent implants, although successful adoption is contingent on achieving an acceptable degradation profile. Plastic strain that is introduced to the device during surgical implantation may influence the resulting degradation behaviour of the implant. In the present work, 3D laser scanning is combined with computer simulation to estimate the level and distribution of surgically-induced plastic strain in a magnesium alloy (ZX10). Subsequently, clinically-relevant pre-strain is shown to influence the rate of corrosion of ZX10 in vitro, indicating the value of such an approach in the design of biodegradable metallic devices under multiaxial loading.