Anomalous in Vitro and in Vivo Degradation of Magnesium Phosphate Bioceramics: Role of Zinc Addition.

In vitro and in vivo degradation behavior and biocompatibility of magnesium phosphate (MgP) bioceramics and the potential role of zinc (Zn) on degradation were compared. Samples were prepared by conventional solid-state sintering at 1200 °C for 2h. Zn-doped MgP (0.5 wt %) showed 50% less degradation than that of pure MgP after immersion into simulated body fluid (SBF) for 8 weeks. Osteoblast-like cell (MG-63) proliferation was evident in MgP ceramics, which was significantly enhanced upon Zn addition. Both Alamar Blue assay and Live/Dead imaging showed the highest cell attachment and proliferation for 0.5 wt % Zn-doped MgP. In vivo biocompatibility of these MgP ceramics were studied after implantation in the rabbit femur. The micro computed tomography (μ-CT) analysis showed that in vivo degradability increased with the increase in the Zn content which is in contradiction to in vitro degradability. Histological evaluation showed large influx of osteoclast cells to the implantation site for Zn-doped MgP samples compared to that of undoped MgP, which is the primary reason of increased degradability of these samples. After 90 days of implantation, large sections of 0.5 wt % Zn-doped MgP samples were replaced by newly formed bones. Fluorochrome labeling showed 78 ± 3% new bone formation for 0.5 wt % Zn-doped MgP ceramics compared to 56 ± 3% for pure MgP samples. Our findings suggest that the addition of Zn in MgP ceramics alters their sintering and degradation kinetics that leads to decreased in vitro degradation, however, when Zn-doped MgP ceramics were implanted in rabbits, higher degradability was observed due to lower Mg ion concentration in the degradation media.