Highly crystalline Mp-1 (TM) hydroxylapatite coating Part I: In vitro characterization and comparison to other plasma-sprayed hydroxylapatite coatings

A novel pressurized hydrothermal post-plasma-spray process, referred to as Mp-1(TM), has been developed to convert the crystalline non-HA and amorphou s components of plasma-sprayed hydroxylapatite coatings back into crystalli ne HA. No detrimental effects are observed on the strength of either the ba se metal or the coating. X-ray diffraction (XRD) and FTIR analysis, surface roughness, shear adhesion strength and calcium solubility testing were con ducted on Calcitite(R) coated samples before and after treatment with this process. Other commercially available coatings were also studied using XRD and solubility testing. Quantitative XRD data show that the MP-1 treatment increases the average crystalline HA content of the Calcitite coating from 77% to 96%, while the amorphous content decreases from 21% to 4%. Other com mercially available dental implant coatings ranged in crystalline HA conten t from 45% to 73%, with amorphous phase content ranging from 29% to 62%. FT IR spectra for treated coatings were significantly more well defined, and s howed an increase in peak separation and intensity. Surface roughness and s hear adhesion strength were not affected by the treatment, In vitro solubil ity testing revealed that for all coatings there is an initial introduction of calcium into solution over the first 2 h of testing; however, the amoun t of calcium dissolved was significantly lower for the MP-1 coating. Under a pH and temperature representative of normal physiologic conditions, the r ate of calcium dissolution for the MP-1 coating was significantly lower tha n that of the other commercial HA coatings. In increasingly acidic conditio ns, the MP-1 coating was compared to the Calcitite coating and was found to have a significantly slower rate of calcium release. The MP-I treatment en hances typical HA coatings by increasing the crystalline HA content at the expense of the plasma-spray-induced soluble phases without a reduction in t he strength of the coating. The resulting coatings exhibit significantly de creased in vitro solubility over a wide range of pH. The results of this so lubility testing suggest that the treated coating may show significantly en hanced in vivo stability, even under the extreme conditions encountered dur ing periods of infection or rigorous detoxification procedures. The signifi cant differences between plasma-sprayed HA coatings reported here underscor e the need for industry and academic researchers to raise the level of disc ourse and understanding of HA coatings. By offering consistent and accurate descriptions of coating compositions and methods of analysis, meaningful c omparisons between different HA coatings can be made.