Modeling the In Vivo Corrosion of Magnesium Alloys: A Biodegradable Alternative to Traditional Orthopedic Implants
Botto, Anna; Jain, Nina; Khan, Haadea; Man, Liza
Magnesium offers a promising alternative to traditional orthopedic implant materials, as it is both biodegradable and osteoconductive. Like traditional implants, magnesium has the mechanical properties necessary to support the surrounding tissue as it heals. Magnesium corrodes when placed into the body, and its osteoconductive properties allow it to be replaced by native bone, eliminating the need for further surgery. The main concern is that pure magnesium implants have been found to degrade too rapidly when studied in vitro. This may lead to catastrophic loss of mechanical integrity as well as potentially lethal production of magnesium ions, hydroxide ions, magnesium hydroxide, and hydrogen gas—all byproducts of the corrosion process. No animal studies using pure magnesium implants have been conducted. However, the magnesium alloy, LAE442, which as been studied in animal models, has been shown to have a slower corrosion rate when compared to pure magnesium in vitro models. Our goal for this study was twofold; we aimed to 1) determine the time required for complete corrosion of both materials after implantation and 2) monitor the concentrations of magnesium ions, hydroxide ions, and magnesium hydroxide as they were affected by the corrosion of both the pure magnesium and LAE442 implants. We developed a two-dimensional axisymmetric model of a rod implanted into the medullary cavity of a human femur using COMSOL Multiphysics 4.3b. Our computational domain consisted of the bone tissue that surrounded the implant. As the implant degraded over time, the boundary between the bone and the implant moved inward toward the axis of symmetry. There was also a corresponding flux of magnesium ions across this boundary, allowing us to model the diffusion and reaction of magnesium ions, hydroxide ions, and magnesium hydroxide in the bone. The main difference between the model of the pure magnesium and that of the LAE442 implant was that the velocity of the moving boundary and the flux of magnesium ions across the implant-bone interface were smaller in the latter model. Since the corrosion rate of the pure magnesium implant was faster than that of the LAE442 alloy, the pure magnesium implant completely degraded in 182 days, compared to 1570 days for the alloy. Due to this faster corrosion rate, there was a greater build-up of magnesium ions and magnesium hydroxide in the pure magnesium model than from the LAE442 alloy after 28 days. For both of these species, the highest concentrations occurred at the point where the line of planar symmetry intersected with the implant-bone interface. The hydroxide ion concentration, however, was lower in the pure magnesium model since the greater build up of magnesium ions lead to a faster consumption of hydroxide ions. The highest hydroxide ion concentration in both models was found at the outer edge of the femur, furthest from the implant. While our model indicated that the decrease in hydroxide concentration was small enough to prevent formation of a toxic acidic environment, our results also indicated that both implants resulted in intolerable concentrations of hydrogen gas. Therefore, neither the pure magnesium nor the LAE442 alloy implants are safe for use in human patients. Further work to develop a slower corroding magnesium alloy is necessary.
In Vivo Corrosion of Magnesium Alloys; Orthopedic Implants