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Numerous studies have shown a three-fold possible effect of calcium phosphate ceramic (CPC) coatings on metallic implants: enhancement of bone tissue growth rates, bonding to bone, and reduction of metal corrosion product release. The two first effects are related to surface reactions of coatings upon implantation. In this study we focus on plasma spray (PS) induced changes in CPC phase compositions and surface areas, and their effect on the in vitro coating stability at physiological pH.
Hydroxyapatite (HA) and β- tricalcium phosphate (β-TCP) PS coatings were obtained from various commercial sources. All of these coatings underwent phase transformations and reduction of specific surface areas due to plasma spray processing. The dissolution rates of all of the PS coatings were found to significantly exceed those of the original HA or β-TCP, in spite of the reduction in specific surface areas. The increase in dissolution rates may be related to plasma spray induced transformations of starting monophases into multiphase coatings containing such components as oxyhydroxyapatite (OHA), α-TCP and tetracalcium phosphate (TTCP).
The multiphase stability depends upon components in the phase mixtures. The in vitro dissolution of monophases and their biphase combinations was studied to assess the influence of each phase component and their combinations. The dissolution rates of the monophase CPCs were found to increase in the order: HA <OHA <β-TCP <α-TCP <TTCP. Dissolution of biphase CPCs, α-TCP/ß-TCP or TTCP/α-TCP, increased with concentration of the more soluble component. Likewise, the dissolution of the PS coatings increased with concentration of the more soluble components. Therefore, PS coatings which contain such metastable components as OHA, α-TCP and TTCP can be expected to be more resorbable than starting HA or ß-TCP.
calcium phosphate, plasma spraying, phase transformation, dissolution
Research Specialist, University of Pennsylvania, Philadelphia, PA
Professor, University of Pennsylvania, Philadelphia, PA