Bioabsorbable & Magnesium (Biodegradable Metal) Implants

What they are. Biodegradable metals, chiefly magnesium with zinc and iron alloys also studied, provide initial fixation and then resorb in vivo. Native tissue gradually replaces the implant, so no removal operation is needed. They are meant to answer the drawbacks of permanent metals, and they are distinct from the bioabsorbable polymers (PLLA, PGA), which are covered separately.

Why magnesium. Its elastic (Young's) modulus is close to that of bone, so it causes less stress shielding than much stiffer titanium or steel. It is biocompatible and osteoconductive, has pro-osteogenic, anti-osteoclastic and anti-inflammatory effects that can enhance bone regeneration, and produces less imaging artefact than permanent metals.

The trade-off. Controlling the corrosion rate is the central challenge. Magnesium can degrade too fast and lose mechanical strength before the fracture has united, which would risk fixation failure; its degradation is variable and site-dependent; and corrosion liberates hydrogen gas, which can form gas pockets. Matching the degradation rate to bone healing through alloying and coatings, while managing hydrogen evolution, is therefore the central engineering goal.

Why adoption has been slow. The engineering problems above, together with the regulatory complexity of a device that is also a biological agent, have delayed widespread clinical use.

The reaction. In vivo magnesium reacts with water: Mg + 2H2O gives Mg(OH)2 + H2. The products are magnesium hydroxide, hydrogen gas (the source of the gas pockets) and a local rise in pH, or alkalinisation.

Why it pits. The magnesium hydroxide forms a partly protective film, but physiological chloride converts it to soluble magnesium chloride. That drives pitting and accelerates breakdown.

Alloying. Alloying slows corrosion by refining the microstructure and adding corrosion-resistant phases. Aluminium-containing alloys are avoided for neurotoxicity. The common orthopaedic alloys are:

• WE43, magnesium-yttrium-rare-earth, the MAGNEZIX alloy
• Magnesium-calcium
• Magnesium-zinc

Coatings. Surface treatments slow the initial corrosion and add bioactivity:

• Magnesium fluoride
• Plasma electrolytic oxidation, which lays down a ceramic oxide layer
• Polymer (PLGA)
• Calcium phosphate or hydroxyapatite

View larger

Iron. Iron alloys are strong but degrade too slowly: months to years, often incompletely, with corrosion products accumulating. Iron is also ferromagnetic, an MRI artefact and safety concern, so it has been explored more for stents than for orthopaedic fixation.

Zinc. Zinc is the 'Goldilocks' metal, with a degradation rate between the too-fast magnesium and the too-slow iron, and no hydrogen-gas evolution. It is biocompatible, being a physiological trace element, but its lower mechanical strength and tendency to creep limit load-bearing use.

Where that leaves magnesium. Against both, magnesium offers bone-like stiffness and osteopromotion, but it degrades fast and forms gas.

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.