Investigation on Mg-Mn-Zn alloys as potential biodegradable materials for orthopaedic applications

Abstract

In fracture management with open reduction and internal fixation with metallic implant, secondary procedure of removal of implant is often required. Such procedure causes additional surgical risks to patients, including anaesthetic risks, wound infection, bone infection, soft tissue adhesion and joint stiffness. The procedure is also costly to the patient and society. If the fixation implant is self-resorbable, the need for secondary surgery will be completely eliminated and the social resources can be saved.

Making use of the corrosion process, metals can be developed into new generation of resorbable (or biocorrodible, biodegradable) implants. An ideal bioresorbable orthopaedic implant should provide adequate mechanical support that matches the bone healing process. The implant should resorb progressively as the bone heals. Many current resorbable materials are biomechanically inferior to conventional metallic implants. Magnesium based alloys are popularly studied because of their mechanical properties and biocompatibility. Implants made of magnesium based alloy are expected to resorb in the human body with no harmful effect.

The major research challenge is to identify an alloy that performs satisfactorily in the following aspects: biocompatibility, degradation rate, hydrogen gas formation (gas product from the reaction between Mg and water), and mechanical strength. In addition, there is no standard evaluation method for the biodegradable alloys. It is because the interaction between the degradable implants and the physiological environment is too complicated to mimic. The in vitro and the in vivo results often mismatch.

This research involved the design and the tests of three Mg based alloys. Zinc (Zn) and manganese (Mn) were chosen as the alloying elements for corrosion resistance and mechanical enhancement. Mg-1Zn-1Mn, Mg-3Zn-1Mn, Mg-5Zn-1Mn (in wt.%) were developed and compared.

The study was divided into three parts: material characterization, in vitro studies, and in vivo (animal) studies. The SEM/EDX confirmed that the surface properties of the alloys were consistent after the surface treatment. From the mechanical test, the yield strengths and the densities of the alloys were found to be close to that of the natural bones. The theoretical calculation showed that the amount of Mn determined the threshold implant mass of the test alloys. The hydrogen evolution test showed that the Mg-1Zn-1Mn was the least corrodible. The elution test showed that the Mg-1Zn-1Mn was the least cytotoxic and the cytotoxicity was affected by the pH changes brought by the alloys. The live cell imaging captured the interaction between the alloys and the cells. The subcutaneous implantation showed that the Mg-3Zn-1Mn formed the smallest gas pocket. In the six-month femoral implantation study (Mg-3Zn-1Mn excluded), the Mg-1Zn-1Mn showed the least volume loss and the steadiest degradation behaviour. It was also found to associate with better bone responses. Concluding from all the results, the Mg-1Zn-1Mn demonstrated better potential to become biodegradable orthopaedic products.

This work evaluated the potentials of the new alloys and proposed some suggestions for the mismatch results. Moreover, quantitative investigation of biomechanical properties, long term degradation behaviour, and toxicity are recommended to be carried out in the future.