Biocompatibility of rapidly solidified magnesium alloy RS66 as a temporary biodegradable metal

Biodegradable magnesium-based alloys are very promising materials for temporary implants. However, the clinical use of magnesium-based alloys is often limited by rapid corrosion and by insufficient mechanical stability. Here we investigated RS66, a magnesium-based alloy with extraordinary physicochemical properties of high tensile strength combined with a high ductility and a homogeneous grain size of ∼1 μm which was obtained by rapid solidification processing and reciprocal extrusion. Using a series of in vitro and in vivo experiments, we analyzed the biodegradation behavior and the biocompatibility of this alloy. In vitro, RS66 had no cytotoxic effects in physiological concentrations on the viability and the proliferation of primary human osteoblasts. In vivo, RS66 cylinders were implanted into femur condyles, under the skin and in the muscle of adult rabbits and were monitored for 1, 2, 3, 4 and 8 weeks. After explantation, the RS66 cylinders were first analyzed by microtomography to determine the remaining RS66 alloy and calculate the corrosion rates. Then, the implantation sites were examined histologically for healing processes and foreign body reactions. We found that RS66 was corroded fastest subcutaneously followed by intramuscular and bony implantation of the samples.No clinical harm with transient gas cavities during the first 6 weeks in subcutaneous and intramuscular implantation sites was observed. No gas cavities were formed around the implantation site in bone. The corrosion rates in the different anatomical locations correlated well with the local blood flow prior to implantation. A normal foreign body reaction occurred in all tissues. Interestingly, no enhanced bone formation could be observed around the corroding samples in the condyles. These data show that RS66 is biocompatible, and due to its interesting physicochemical properties, this magnesium alloy is a promising material for biodegradable implants.