Bioabsorbable & Magnesium (Biodegradable Metal) Implants

• The corrosion reaction. In vivo magnesium reacts with water: Mg + 2H2O gives Mg(OH)2 + H2 - magnesium hydroxide plus hydrogen gas (the source of the gas pockets), with local alkalinisation (a rise in pH). The Mg(OH)2 forms a partly protective film, but physiological chloride converts it to soluble magnesium chloride, driving pitting and accelerating breakdown.
• Alloying to slow it. Alloying refines the microstructure and adds corrosion-resistant phases: WE43 (magnesium-yttrium-rare-earth, the MAGNEZIX alloy), Mg-calcium and Mg-zinc are the common orthopaedic alloys (aluminium-containing alloys are avoided for neurotoxicity).
• Coatings to slow it. Surface treatments - magnesium fluoride, plasma electrolytic oxidation (a ceramic oxide layer), polymer (PLGA) or calcium-phosphate/hydroxyapatite coatings - slow the initial corrosion and add bioactivity.

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• Iron. Iron alloys are strong but degrade too slowly (months to years, often incomplete, with accumulating corrosion products), and iron is ferromagnetic (an MRI artefact/safety concern) - so it has been explored more for stents than orthopaedic fixation.
• Zinc. Zinc sits in the middle - a degradation rate between the too-fast magnesium and the too-slow iron, with no hydrogen-gas evolution and good biocompatibility (zinc is a physiological trace element) - but its lower mechanical strength and tendency to creep limit load-bearing use.
• The trade-off. So across the biodegradable metals: magnesium = bone-like stiffness and osteopromotion but fast and gas-forming; zinc = ideal degradation rate but weak; iron = strong but too slow and ferromagnetic.

Related pages: Bioabsorbable Materials covers the PLLA/PGA polymers this class was developed to replace - weaker, and prone to the sterile inflammatory osteolysis that magnesium has so far avoided. Titanium Alloys is the comparator in every trial on this page, and Screw Biomechanics with Implant Fracture Biomechanics explain why a bone-like elastic modulus matters and why a small magnesium screw cannot be asked to carry load. Corrosion Mechanisms is the same electrochemistry read as a failure mode rather than a design feature - here corrosion is the intended behaviour, which is the conceptual inversion worth holding onto; Wear Mechanisms and Tribology and Wear cover the debris problem that does not arise when the implant is meant to disappear. MRI Metal Artefact Reduction is the technique magnesium partly makes unnecessary, and the reason the three-year MRI comparison favoured it. Bone Healing sets the timeline the degradation rate must be matched to - the whole engineering problem is keeping strength until union. Clinically the material lives in Hallux Valgus surgery, where every randomised comparison has been done, Scaphoid Fractures and the carpus, where gas formation is the reported side effect, and Osteochondral Lesions of the Talus with Osteochondral Defects of the Knee, where a fixation device that vanishes is most attractive.