| Magnesium phosphate based ceramic implants and cements are currently under development and have received much attention to challenge the current standards for bone grafting materials. Compared to current calcium phosphate implants, magnesium phosphate implants display higher levels of resorption in vivo while maintaining high biocompatibility and osteoconductivity and low inflammatory response. The magnesium phosphate ceramics under development include dense pellets as well as cement putty, and fiber reinforced cement composite embodiments. In pellet form, amorphous tri-magnesium phosphate pellets displayed higher solubility than crystalline tri-magnesium phosphate while inducing rapid mineralization on the pellet surface which aided high osteoblast cytocompatibility of the amorphous pellet. These crystalline and amorphous tri-magnesium phosphates were therefore explored as precursor materials for a variety of cementing reactions. Amorphous, semi-crystalline and crystalline tri-magnesium phosphate precursors were correspondingly reacted with an ammonium phosphate solution to explore the effect of tri-magnesium phosphate crystallinity on the cementing reaction. The amorphous and semi-crystalline tri-magnesium phosphate powders were highly reactive, resulting in mechanically weak cements, while the crystalline tri-magnesium phosphate powder reacted efficiently with the reacting solution and were mechanically strong following reaction completion reaction while also displaying a neutral pH during incubation in saline solution. This crystalline tri-magnesium phosphate was next evaluated with solutions of ammonium, potassium and sodium phosphate to explore the effect of each reacting salt solution on the cement reaction and product. The sodium and potassium phosphate solutions resulted in long setting times and poor mechanical resilience due to a lack of reaction completion while the ammonium phosphate solution resulted in mechanically resilient cement exhibiting clinically relevant setting times. This magnesium ammonium cement formulation was then further modified by the addition of soluble mannitol sugar crystals, or wet-spun degradable polycaprolactone fibers to evaluate the influence of added porosity or fiber reinforcement. Mannitol inclusion improved the porosity while reducing the mechanical strength. Polymeric fibers addition increased the toughness of the cement and, when removed, led to significant increase in macroporosity contributing to improved cellular infiltration. Based on the reults of the compositions studied, magnesium ammonium phosphate cements containing amounts of mannitol or more significant amounts of degradable fiber serving as initial reinforcement as well as eventual pore formers appear to be promising formulations to explore in the future. |
