In vivo degradation performance of micro-arc-oxidized magnesium implants: A micro-CT study in rats

Biodegradable Mg alloys are of great interest for osteosynthetic applications because they do not require surgical removal after they have served their purpose. In this study, fast-degrading ZX50 Mg-based implants were surface-treated by micro-arc oxidation (MAO), to alter the initial degradation, and implanted along with untreated ZX50 controls in the femoral legs of 20 male Sprague-Dawley rats. Their degradation was monitored by microfocus computed tomography (μCT) over a total observation period of 24weeks, and histological analysis was performed after 4, 12 and 24weeks. While the MAO-treated samples showed almost no corrosion in the first week, they revealed an accelerated degradation rate after the third week, even faster than that of the untreated ZX50 implants. This increase in degradation rate can be explained by an increase in the surface-area-to-volume ratio of MAO-treated implants, which degrade inhomogeneously via localized corrosion attacks. The histological analyses show that the initially improved corrosion resistance of the MAO implants has a positive effect on bone and tissue response: The reduced hydrogen evolution (due to reduced corrosion) makes possible increased osteoblast apposition from the very beginning, thus generating a stable bone-implant interface. As such, MAO treatment appears to be very interesting for osteosynthetic implant applications, as it delays implant degradation immediately after implantation, enhances fracture stabilization, minimizes the burden on the postoperatively irritated surrounding tissue and generates good bone-implant connections, followed by accelerated degradation in the later stage of bone healing.