Av. Burgess et al., Highly crystalline Mp-1 (TM) hydroxylapatite coating Part I: In vitro characterization and comparison to other plasma-sprayed hydroxylapatite coatings, CLIN OR IMP, 10(4), 1999, pp. 245-256
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.