115. Additive Manufacturing of Degradable Metal Implants for Craniofacial Reconstruction

PURPOSE: Implantable metal plates and screws are essential tools for craniofacial reconstruction as they provide stable bony reduction to facilitate osteosynthesis. However, once bone healing is complete, they become superfluous and can lead to complications such as infection, bone loss or uncomfortable palpability. In pediatric patients, metal implants may impede subsequent bone growth and may require removal. Although resorbable materials like poly(L-lactide-co-glycolide) are clinically used, their limited strength compared to metallic implants restricts their application. Thus, a degradable metal implant, which would be absorbed after bone healing is completed, presents an appealing solution. Combining Iron (Fe, slow degradation rate) and Magnesium (Mg, fast degradation rate), which both corrode under physiologic conditions, in a composite structure could be a viable path to achieve degradation tunability for metal implants. However, the major obstacle to the fabrication of Fe-Mg plates is the miscibility gap of these metals, where the melting point of Fe is above the boiling point of Mg, obviating traditional alloy manufacturing approaches. Therefore, in this study we utilize a novel additive manufacturing approach to create Fe-Mg composite implants as an initial step towards designing degradable craniofacial metal implants. METHODS: The designed Fe-Mg composite was fabricated from a powder mixture containing approximately 20vol% Mg using the Formalloy direct energy deposition printer. Small plates (10 x 2 x 0.6mm) with three holes (0.2mm diameter) were printed and tested in in vitro and in vivo experiments to evaluate degradation rate and surface corrosion. Fe-only plates manufactured similarly, and commercially available Titanium plates of comparable shape and weight were tested as controls. RESULTS:In vitro, plates were immersed in the Hank’s balanced salt solution at 37°C to recapitulate the physiological conditions for 24 weeks, demonstrating faster decomposition in Fe-Mg composite group compared to Fe-only group. After 6, 12 and 24 weeks of in vivo subcutaneous implantation, both Fe-Mg composite and Fe-only plates presented increased rust microparticle accumulation in the surrounding tissue capsule, especially the Fe-only group. In contrast, the Titanium plates remained inert and developed a thin capsule. Histologic study verified the iron deposition in the capsule in both Fe-Mg composite and Fe-only groups, with no evidence of off-target toxicity to liver, kidney, or other organs. Hematological and serum biochemical analysis, including Fe and Mg content, showed no differences between groups or time points. CT scans correlated the degradation rates between in vivo and in vitro, while scanning electron microscopy provided insights into surface corrosion and effects of the biological media on the metallic surfaces. CONCLUSION: The custom-designed Fe-Mg metallic composite plates demonstrated evidence of degradation without any local or systemic metal toxicity. By adjusting the volume fraction of Fe and Mg, as well as the geometry and porosity of the composition, the absorption rate of the biodegradable metallic implant can be tailored to meet specific clinical needs.