Comparison between three in vitro methods to measure magnesium degradation and their suitability for predicting in vivo degradation.

A lot of research has been done in the field of magnesium-based implant material. This study is focused on finding an explanation for the large disparity in results from similar experiments in literature. The hypothesis is that many different measurement protocols are used to quantify magnesium degradation and this leads to inconsistent results. Cylindrical, pure magnesium samples were used for this study. The degradation took place in revised simulated body fluid at 37°C. Hydrogen evolution was measured to quantify the degradation. Two commonly used experimental protocols were examined: static conditions and a fluid changing method. For static testing, the samples stayed in fluid. For the fluid changing method, the fluid was changed after 2 and 5 days of immersion. In addition, a new method with continuous fluid flow was established. After an initial phase, the results confirm that for all three methods, the degradation behavior differs strongly. The static condition results in a very slow degradation rate. The fluid change method leads to a similar behavior like the static condition except that the degradation was speeded up after the fluid changes. The continuous degradation is linear for a long period after the initial phase. In comparison with in vivo degradation behavior, the degradation process in continuous flow shows the best fitting. The accumulation of degradation products, especially the increasing pH value, has a strong inhibiting effect. This cannot be observed in vivo so that a constant experimental environment realizable by continuous flow is more suitable for magnesium-based implant material testing.