Biodegradable magnesium implants (also referred to as bioabsorbable magnesium implants, resorbable magnesium implants, or degradable magnesium implants) are temporary surgical implants fabricated from magnesium and its alloys that gradually dissolve within the body's physiological environment during and after bone healing. Unlike conventional permanent implants made from titanium or stainless steel, a bioabsorbable device is designed to degrade once the bone has healed, eliminating the need for a secondary surgical removal procedure.
Magnesium is the fourth most abundant cation in the human body and an essential component of bone mineral, contributing to its inherent biocompatibility. Its elastic modulus (41–45 GPa) is considerably closer to that of cortical bone (10–30 GPa) than conventional permanent metallic biomaterials such as titanium alloys (~110 GPa) or stainless steel (~200 GPa), reducing the stress shielding effect that can inhibit bone remodelling and lead to bone resorption. Beyond passive compatibility, magnesium ions released during degradation have been shown to actively promote osteogenesis, stimulate angiogenesis, and modulate immune responses in the bone microenvironment.
The principal clinical forms include screws, pins, plates, and intramedullary nails used in fracture fixation, as well as bioresorbable coronary scaffolds. The first regulatory-approved resorbable metallic orthopaedic implant—the MAGNEZIX® compression screw (Syntellix AG, Germany)—received CE marking in 2013. As of 2026, multiple companies across Europe, Asia, and North America hold approved or breakthrough-designated products, targeting markets in orthopaedic trauma and spinal surgery valued in excess of USD 9 billion.
Despite growing clinical adoption, challenges related to corrosion rate control, stress corrosion cracking (SCC), hydrogen gas evolution, and regulatory standardisation continue to restrict broader deployment. An active area of research involves developing quantitative design parameters—such as SCC stress thresholds—to support safe implant design beyond comparative susceptibility ranking.
History
The clinical exploration of biodegradable magnesium implants originated in the late nineteenth century, driven by the metal's unique resorption properties. In 1878, physician Edward C. Huse first utilized magnesium wires as ligatures to arrest bleeding from vessels. Subsequent pioneering work was conducted by Austrian surgeon Erwin Payr, who, beginning in 1900, engineered absorbable magnesium tubular stents to facilitate vascular anastomosis and nerve repair. Similarly, in 1906, Belgian surgeon Albin Lambotte conducted the first documented clinical trial of magnesium in orthopedics, utilizing a pure magnesium plate and steel screws to stabilize a pediatric tibia-fibula fracture. However, these early clinical applications were severely limited by magnesium's high in vivo corrosion rate. The rapid degradation not only resulted in a premature loss of the implant's mechanical integrity before tissue healing could occur, but also drove a localized electrochemical reaction that released excessive volumes of hydrogen gas, forming extensive subcutaneous gas cavities. Interest was comprehensively revived from the 1990s and particularly during the 2000s as advances in alloy design, surface engineering, and characterisation techniques made controlled degradation achievable. Research expanded in two principal directions: (1) orthopaedic fracture fixation devices; and (2) cardiovascular resorbable scaffolds. The regulatory milestone came in 2013, when Syntellix AG (Hannover, Germany) received CE marking for its MAGNEZIX® compression screws—the first resorbable metallic orthopaedic implant approved for human use worldwide. The same year, Biotronik received CE marking for the first generation of its absorbable magnesium coronary stent. K-MET screws (K-MET Co., Ltd., Republic of Korea) received approval from the Korean Food and Drug Administration in 2015 for distal radius fracture repair. By 2016, MAGNEZIX® had been used in more than 25,000 patients in a single year. Ian Technology (China) obtained CE certification for its high-purity magnesium (HP-Mg) bone screw in 2020, and Bioretec Oy (Finland) received US FDA clearance for its RemeOs™ trauma screw in March 2023 and CE marking in January 2025.
Properties of magnesium relevant to bone implantation
Mechanical properties
The elastic modulus of magnesium alloys (41–45 GPa) falls between that of cortical bone (10–30 GPa) and cancellous bone (~0.1–5 GPa), representing a substantially better mechanical match than titanium or steel. This reduces stress shielding—where a much stiffer implant bears a disproportionate share of load, depriving the adjacent bone of mechanobiological stimulation needed for remodelling.
The density of magnesium (~1.74 g/cm³) is close to that of cortical bone (~1.75–2.0 g/cm³), giving implants a high strength-to-weight ratio (tensile: ~158 kN·m/kg) and, in orthopaedic contexts, reducing implant-related inertia during patient activity. Tensile and compressive strengths of processed alloys are broadly adequate for temporary load-sharing fixation (typically 150–300 MPa depending on composition and processing route), though load-bearing applications such as femoral fixation and spinal instrumentation remain challenging owing to the difficulty of reliably achieving yield strengths above 200 MPa in all product geometries.
Biocompatibility
Approximately half of the body's total magnesium resides in bone, and Mg²⁺ ions are involved in hundreds of enzymatic reactions. The primary degradation products—Mg²⁺ ions and magnesium hydroxide—are not inherently cytotoxic and are excreted via the kidneys. Clinical studies have generally shown that serum magnesium and calcium concentrations are not significantly altered by the presence of degrading Mg implants.
Beyond passive biocompatibility, Mg²⁺ ions released during degradation promote new bone formation through stimulation of osteoblast differentiation and migration, encourage angiogenesis in regenerating bone, and exert immunomodulatory effects via macrophage polarisation towards a pro-healing (M2) phenotype. A 2024 study in Nature Biomedical Engineering reported that Mg²⁺ ions from resorbable screws upregulated osteocalcin expression in human mesenchymal stem cells by 3.2-fold, accelerating callus maturation in rat femoral fractures. These osteostimulatory properties are a distinctive advantage over biologically inert permanent metallic implants.
Biodegradation mechanism
In the chloride-rich environment of physiological fluids, magnesium degrades electrochemically. The predominant overall reaction is:
Mg + 2H₂O → Mg(OH)₂ + H₂↑
The magnesium hydroxide (brucite) film that forms on the surface is partially protective but susceptible to dissolution by chloride ions, which convert it to the more soluble MgCl₂. This is accompanied by the local evolution of hydrogen gas. Uncoated pure magnesium degrades at rates of roughly 2–4 mm/year in simulated body fluid, which is too rapid for most clinical applications. The degradation rate is influenced by alloy composition, microstructure, second-phase distribution, surface condition, local pH, protein adsorption, and ionic concentrations in the surrounding physiological medium.
Alloy systems
The performance of a magnesium implant depends critically on alloy composition, which governs mechanical strength, corrosion rate, and biological response. Commercial magnesium alloys are broadly classified into Mg–Al, Mg–Ca, Mg–Zn, Mg–Mn, Mg–Sr, Mg–Zr, Mg–RE (rare-earth), and multicomponent systems. For biomedical use, aluminium-free and rare-earth-free compositions have attracted increasing interest on safety grounds.
Pure magnesium
High-purity magnesium (HP-Mg, ≥99.9%) has been investigated and clinically applied because the absence of alloying elements minimises cytotoxicity concerns. HP-Mg exhibits relatively uniform corrosion, and Ian Technology (China) received CE certification in 2020 for an HP-Mg bone fixation screw. Its principal limitations are modest mechanical strength and a relatively rapid degradation rate that is difficult to tailor without alloying.
Mg–Al–Zn alloys
Alloys of the AZ series (e.g., AZ31, AZ61, AZ91D) were among the first investigated due to their well-established room-temperature mechanical properties and industrial familiarity. Aluminium has the highest solid solubility in magnesium (~12.7 wt%), enabling solid-solution strengthening and precipitation of the β-Mg₁₇Al₁₂ phase. However, concerns about potential cytotoxicity and neurotoxicity of aluminium ions accumulating locally near the implant have substantially limited clinical enthusiasm for this series in long-term biodegradable applications. More recent work on the Biotronik Freesolve™ third-generation scaffold has revealed that the speciation and systemic mobilisation of aluminium from Mg–Al alloys are critically dictated by microstructural design, with peak-aged alloys favouring localised particulate release over systemic ionic exposure.
Mg–Ca binary alloys
Calcium is an essential element in the human body and the principal mineral constituent of bone, making Mg–Ca alloys conceptually appealing for physiological compatibility. Calcium refines grain structure and promotes corrosion-resistant Mg₂Ca intermetallic formation. Mechanical properties and corrosion resistance are best when calcium content is kept below 1 wt%; at higher levels, the volume fraction of the cathodic Mg₂Ca phase increases, accelerating galvanic corrosion. During in-vivo breakdown, calcium phosphate precipitates form that promote bone mineralisation. Binary Mg–Ca alloys were among the earliest truly biocompatible candidate systems studied and form the conceptual basis for many ternary compositions.
Mg–Zn binary alloys
Zinc is an essential trace element involved in bone formation and enzyme function. Addition of Zn to magnesium increases yield strength (through solid-solution and precipitation hardening) and reduces corrosion rate by forming protective surface films; Mg–Zn alloys exhibit high biocompatibility and are considered among the clinically safest Mg-based systems. Maximum elongation (~15.8%) is typically achieved at Zn concentrations up to about 4–6 wt%. Mg–Zn alloys have been evaluated for interference screws and fracture fixation pins, and the class forms the binary backbone of the widely studied Mg–Zn–Ca ternary family.
Mg–Ca–Zn ternary alloys
Ternary Mg–Ca–Zn alloys have attracted substantial research interest as they combine the physiological compatibility of both calcium and zinc without either aluminium or rare-earth elements. Microstructural studies show the formation of Mg₂Ca and Mg₆Ca₂Zn intermetallic compounds. In-vitro cytotoxicity studies have demonstrated cell viability of 95–99% for compositions in the range of 0.5–1 wt% Ca and 1–4 wt% Zn, establishing that these are suitable for biomedical use. Optimal compositions for the balance of corrosion resistance and mechanical strength are considered to lie in the range 0.3–0.8 wt% Ca and 0.5–4 wt% Zn. Dilute compositions such as Mg–0.6Zn–0.5Ca (ZX00) and ZXM100 have been specifically engineered for resorbable cardiovascular and orthopaedic applications; ZX00 demonstrated significantly higher fibroblast viability than WE43 in vitro, and degraded through uniform rather than localised corrosion mechanisms, retaining superior ductility after five months of in-vivo implantation.
Detailed microstructural characterisation of the Mg–0.45Zn–0.45Ca alloy in heat-treated and thermo-mechanically processed conditions has shown that processing route significantly influences grain size, second-phase distribution, and corrosion behaviour—underscoring the importance of post-processing for achieving clinically adequate degradation rates and mechanical properties.