Surface characteristics and dissolution behavior of plasma-sprayed hydroxyapatite coating

One of the most important concerns with the clinical use of plasma-sprayed hydroxyapatite (HA) coatings is the resorption of the coating, and dissolution at neutral pH is one of the two major resorption mechanisms. In this study, highly crystalline pure HA powders were atmospherically plasma sprayed using various parameters. Dissolution of both HA powders and coatings was measured using a calcium ion meter. Surface characteristics, including phase, morphology, and roughness, were compared for the coatings before and after dissolution. Pulverized HA coatings exhibited significantly higher dissolution compared with the same quantity of feedstock HA powders because of the decreased crystallinity and fine crystal size of the coating. Furthermore, the dissolution decreased with the crystallinity of the coating. Dissolution of HA coatings did not show much difference with respect to the coatings in the initial stage of immersion (4 h). However, dissolution of all coatings reached saturation in a fresh physiological solution. The saturation values were much lower compared with their counterparts in the form of powders, which may imply the stability of HA coatings in long-term use. In addition to crystallinity, the particle melting status in the coatings, i.e., the volume of nanocrystals, and porosity, was found to be another important factor for the dissolution of the HA coating. X-ray diffraction patterns of HA coatings indicated the complete dissolution of impurity phases and amorphous phase after the coatings were immersed in the solution for 4 days. Coatings sprayed at lower power (27.5 kW) exhibited a pattern of crystalline HA whereas coatings sprayed at higher power (42 kW) exhibited a pattern of bone apatite. Surface morphologies showed preferential dissolution of amorphous phase in all coatings accompanied with precipitation of bone apatite observable for coatings sprayed at higher power. Surface roughness measured after the dissolution studies increased for the two coatings sprayed at lower power level but decreased for coatings sprayed at higher power level. This decrease is attributed to the better match in solubility characteristics between the fine crystals and the amorphous calcium phosphate within the coating