An intramedullary implant model in the canine femora was developed to evaluate the mechanical and histological responses between cancellous bone and plasma-sprayed hydroxyapatite coatings (HACs) on ti-6A1-4V implants, with 12- and 24-week follow-ups. HACs of different thicknesses were investigated. Results of the mechanical testings revealed that after 24 weeks of implantation, the mean shear strength (2.49 +/- 0.12 MPa) of the 50 microns HACs was significantly higher (p < 0.05) than that of the 200 microns HACs (1.44 +/- 0.19 MPa). However, using backscattered electron images (BEIs) throughout all the implant periods, no substantial histological variations in the extent of new bone apposition between the two HACs were observed. Occasionally, solution-mediated disintegration of the 50 microns HAC was found 24 weeks postimplantation. Histomorphometric studies from the BEIs demonstrated that for both HACs the percentage of the direct HAC-cancellous bone contact was approximately 50% at 12 weeks and 75% at 24 weeks. After the mechanical tests, the 200 microns HACs had fracture sites either inside the coating layers or at the HAC-titanium interfaces, which might explain why the mechanical performance of the 200 microns HACs was inferior to that of the 50 microns HACs even though both HACs had the same histological behaviors.

This study was undertaken to evaluate the effect of coating characteristics on the mechanical strengths of the plasma-sprayed HA-coated Ti-6Al-4V implant system both in vitro and in vivo. Two types of HA coatings (HACs) with quite different microstructures, concentrations of impurity-phases, and indices-of-crystallinity were used. In vitro testings were done by measuring the bonding-strength at the Ti-6Al-4V-HAC interface, with HACs that had and had not been immersed in a pH-buffered, serum-added simulated body fluid (SBF). The shear-strength at the HAC-bone interface was investigated in a canine transcortical femoral model after 12 and 24 weeks of implantation. The results showed a bonding degradation of approximately 32% or higher of the original strength after 4 weeks of immersion in SBF, and this predominantly depended on the constructed microstructure of the HACs. After the push-out measurements, it was demonstrated that the HACs with higher bonding-strength in vitro would correspondingly result in significantly higher shear-strength at each implant period in vivo. Nevertheless, there were no substantial histological variations between the two types of HACs evaluated. The most important point elucidated in this study was that, among coating characteristics, the microstructure was the key factor in influencing the mechanical stability of the HACs both in vitro and in vivo. As a consequence, a denser HAC was needed to ensure mechanical stability at both interfaces.

Conventional bioactive glasses, in bulk form, are being considered as biomaterials in prosthetic applications. In this study, a new attempt was made to coat bioactive glasses on Ti-6A1-4V by plasma spraying. This method will coat the bioactive glass coatings (BGCs) onto metal substrate, potentially combining the excellent mechanical strength of metal and biocompatibility of bioactive glass. Analysis by X-ray diffractometry (XRD) of the BGCs, revealed that the amorphous structure of glass was preserved. BGCs were soaked in simulated body fluid (SBF) to evaluate their properties in vitro. After soaking in SBF for 1 day, precipitation of fiber structure was observed on the surface of the BGCs. After 2 and more days, the surface of the BGCs was completely covered with precipitates. The precipitates, identified as the apatite phase by XRD, contained carbonate and hydroxyl functional groups detected by Fourier transform IR reflection (FTIR) spectroscopy. After soaking for 16 days, a thin layer of about 10 μm, rich in calcium and phosphorus but poor in silicon, was observed on the surface of the BGCs. The composition of the CaP rich layer was consistent with the apatite structure identified by various methods, but the apatite layer was significantly thicker than reported in bulk form. The formation of an apatite phase surface has been suggested to be indicative of biocompatibility. All findings in this study indicated the formation of apatite on the surface of plasma-sprayed BGCs, and this material is expected to be biocompatible in vivo.

The successful use of the plasma-sprayed HA-coated Ti-6AI-4V system requires strong adhesion between the ceramic coating and the underlying metal substrate. The aim of this study was to evaluate the bond strength at the HA coating (HAC)/Ti-6AI-4V interface, for specimens that had and had not been subjected to immersion in a pH-buffered, serum-added simulated body fluid (SBF). Moreover, coating characteristics affecting the mechanical stability after having been immersed in SBF were clarified. The results showed that bonding degradation of approximately 25–33% of the original strength was measured after immersion in SBF, and that this predominantly depended on the characteristics of the HAC and the period of immersion. Since the surface morphologies of HACs have dissolved in the SBF, it is suggested that the interlamellar structure of the HAC was weakened and, therefore, the bond strength degraded. As both the crystallinity and impurity phases of the HAC increased with immersion time, it can be concluded that the dissolution of the HAC resulting from the initial microstructure has overtaken that of the coating crystallinity and phase purity. A denser microstructure is required to ensure a satisfactory HAC/Ti-6AI-4V interface.

With three kinds of plasma-sprayed hydroxyapatite coatings (HACs) prepared, the objectives of this study were (1) to establish the calibration methods for quantitatively measuring the concentration of impurity phases and the degree of crystallinity of the HACs, and (2) to explore the effects of postheat treatments at various temperatures in vacuo on the changes of phases and crystallinity of the HACs. By the internal standard method used, the concentrations of impurity phases, such as alpha-TCP, beta-TCP, and TP, of the assprayed HACs were significantly higher than those measured by the direct intensity-ratio method, and the CaO phase was lower than the direct intensity-ratio method. When the HACs were heat treated in the temperature interval 630-850 degrees C, the concentrations of impurity phases obviously decreased, and the coating crystallinity apparently increased. After annealing at 850 degrees C, an HAC consisting of at least 95% crystallinity with few impurity phases was obtained. As the annealing temperatures in the interval 850-1000 degrees C were applied, however, the HA phase seriously decomposed, resulting in the appearance of a large number (higher than 20 wt%) of impurity phases in the HACs. This work suggests that the optimum heat treatment conditions in vacuo for maximizing crystallinity and minimizing impurity phases of the HACs do not occur at the same temperature.

Plasma-sprayed hydroxyapatite coating (HAC) on a bioinert metal substrate was used clinically to increase the fixation of an orthopaedic implant. This study aimed to clarify in vitro various characteristics of plasma-sprayed HACs, and to evaluate in vivo the effect of coating characteristics on the osteoconductivity of HACs. Three different HACs on Ti-6A1-4V substrate were prepared by varying the plasma spraying parameters; these were then characterized in detail. Subsequently, in the cortex of canine femur, a quantitative histologic evaluation was performed. This was done to determine the osteoconductivity of HACs, represented as the new bone healing index (NBHI), after 2, 4, 6, and 12 weeks of implantation. The results in vitro demonstrated that the microstructure, phase composition, crystallinity, OH-ion content, and calcium to phosphorus molar ratio of the HACs varied with the spraying parameters. The HAC with denser microstructure and less thickness showed a higher bonding strength at the HAC-Ti-6A1-4V interface. However, this denser HAC underwent adverse biological degration in terms of NBHI after 12 weeks of implantation because of other characteristics of the coating. Based on the results of the study, it can be concluded that an HAC with both high bonding strength and good osteoconductivity is difficult to acheive.

Plasma-sprayed hydroxyapatite coated (HAC) 50 and 200 mum thick on Ti-6Al-4V cylinders was transcortically implanted in the femora of canines to evaluate in detail the effect of coating thickness on the push-out shear strength and failure mode examined under scanning electron microscope after the periods of 4, 6, 8, and 12 weeks. The HAC coating exhibited higher shear strength at 50 mum than at 200 mum. Its failure mode was conclusively at or near the HAC-bone interface, and the slight attack of body fluid had not degraded the implant to the extent that failure occurred at the HAC-Ti alloy interface after 12 weeks of observation. For 200 mum-HAC, failure was found at the HAC-bone interface, inside the HAC lamellar splat layer and at the HAC-Ti alloy substrate interface, depending on the period of implantation. It was also deduced that the variation of failure mode of 200 mum-HAC with time could not be accounted for by the attack of body fluid alone; the degradation must be a synergetic adverse result of residual stress in the HAC and the attack of body fluid. (C) 1993 John Wiley & Sons, Inc.