Effect of potassium and magnesium doping on mechanical properties and in vitro degradation behavior of calcium polyphosphate

Porous crystalline calcium polyphosphate (CPP) is under investigation as a candidate bone substitute/augmentation material including incorporation in implants intended for repair of osteochondral defects. Previous studies of biphasic implants (i.e., cartilage-CPP constructs) for osteochondral defect repair have shown that porous CPP has the required features for this application including the bone substitute portion of the biphasic implant, but the porous CPP degradation rate is lower than preferred. This study investigated the effect of doping with MgCO₃, MgCl₂, K₂CO₃, or KCl at a molar ratio of M/Ca = 0.02 on properties and in vitro degradation behavior of CPP. Doping with magnesium or potassium resulted in changes in the crystallization and melting temperatures, which required adjustment of the sintering conditions for forming samples of the desired porosity level. This, in turn, resulted in higher compressive and diametral compressive (i.e., tensile) strengths of the porous-doped CPP samples compared with undoped CPP prepared to a given porosity level. For samples prepared in this study, the chemical degradation rate of porous Mg-doped CPP samples was the fastest while K doping resulted in a lower degradation rate than undoped CPP.