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Plasma-sprayed HA coating (HAC) 50 and 200 microm thick on Ti6Al4V cylinders was transcortically implanted in the femora of canines. Push-out testing of implant-bone interfaces showed that the HAC coating exhibited higher shear strength at 50 microm coating than 200 microm one. The plasma-sprayed HACs were exhibited compressive residual stresses and the thicker HAC exhibited higher residual stress than that of the thinner HAC. Due to the structure for 50 and 200 microm implants were the same, meaning similar cohesive strength of the lamellar splats. And, there was no difference in the physiological environment; hence the difference of the shear strength for the 50 and 200 microm-HAC implants could best be attributed to the compressive residual stress existed in the HA coating.

The aim of the study was to compare the pin-bone interface microstructural characteristics of hydroxyapatite-coated (HAC) and stainless steel Schanz screws after 2, 4 and 6 months of implantation in a sheep model. The microstructure and composition of the hydroxyapatite coating were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Twelve coated and 12 uncoated screws were implanted into both femora of three sheep, each sheep receiving eight screws. Specimens of polished bone with screws were examined with SEM and light microscope for morphometric analyses. The HAC was approx. 40 microm thick, the grain size ranged from 5 to 40 microm, with pores less than 20 microm. The atomic ratio of Ca/P was 1.62. SEM showed that the bone-implant contact was better with HAC than with uncoated implants. The ingrowth of the bone in the HAC was clearly seen. Morphometric analysis showed good bone-implant contact in 65.1 (+/-24.6)% in the HAC and 32.0 (+/-23.3)% in the uncoated group (p0.001). Although the percentage of good contact increased with time for both groups, it was significantly higher for HAC screws. Our investigation demonstrated a time dependent improvement of implant-bone contact of the HAC compared to standard stainless steel implants in the chosen experimental conditions.

Titanium alloys are hoped to be used much more for applications as implant materials in the medical and dental fields because of their basic properties, such as biocompatibility, corrosion resistance and specific strength compared with other metallic implant materials. Thus, the Ti-6Al-7Nb alloy that has recently been developed for biomedical use, that is, primarily developed for orthopaedic use, is to be studied in this paper, for application in dental implants. The biocompatibility test in vivo was carried out in dogs and the osseointegration was verified through histological analysis of the samples of the Ti-6Al-7Nb alloy with and without hydroxyapatite coating that were inserted in the alveoli. Within the controlled conditions the samples did not show any toxic effects on the cells. © 2001 Kluwer Academic Publishers

The bone implant interface in stainless steel pins coated with hydroxyapatite, used in a monoaxial fracture external fixation system was examined. The pins were transversally inserted into sheep tibial diaphyses where a defect was created, and they were loaded for 6 weeks. Uncoated pins were implanted as control. The microscopic relation between bone and implant was quantified through image analysis: the residual thickness of the hydroxyapatite coating, pin-bone contact surface and bone ingrowth value in between the threads were measured. The bone tissue at the interface appeared regularly mineralized and viable both in the implants of coated pins and in the control uncoated ones. The ceramic coating showed a slight and not statistically significant increase in thickness. The ceramic coated pins presented contact with bone higher than the uncoated pins (75.6±20.0 versus 47.5±19.4); they also induced a higher bone ingrowth (86.6±22.4 versus 78.7±13.5). Both differences are not statistically significant, but suggestive of a trend. The authors concluded that the hydroxyapatite coating of the pins might improve the performance of external fixators, by favouring bone apposition and reducing rate of loosening.

Thermal wave interferometry is a non-destructive inspection technique that has been used successfully for the assessment of coating thickness. The purpose of this paper is to present an evaluation of the suitability of the technique for the measurement and characterization of hydroxyapatite coatings on commercially available prosthetic hip stems. The results indicate a good correlation between the coating thickness values obtained using the interferometry technique and measurements made using optical microscopy of sectioned stems. In addition the results confirm that thermal wave interferometry can be used to accurately assess the changes in coating thickness on samples with a complex geometry.

Hydroxyapatite-coated coupons were tested in various physiological media. Immersion in Ringer's solution showed that heat-treated coatings displayed a weight gain but the assprayed coatings underwent a weight loss. Dissolution of the coating was measured by weighing the specimen before and after ageing. Immersion of hydroxyapatite coatings showed the appearance of small spheres that were identified by X-ray diffraction as hydroxyapatite. Changes in coating morphology were detected and the coating degradation mechanisms are discussed. This paper thus looks at the morphology, composition, crystallinity and dissolution of hydroxyapatite coatings aged in Ringer's solution.

A method of producing layered, adhesively bonded hydroxyapatite coatings is described. The coatings have a flexible polymeric interlayer which can improve load transfer from the implant to the bone, and a hydroxyapatite-rich surface which can provide mechanical interlocking with the bone. The coatings have been characterized using a number of methods including optical and electron microscopy, mechanical testing and finite element analysis. Some ways in which the coatings could be used to produce orthopaedic implants with improved mechanical and biological compatibility are discussed.

Hydroxyapatite coatings have been used for many years on dental and prosthetic implants to provide a biocompatible surface for long-term fixation of the implant to bone. In this study two thermal spraying processes, air plasma spraying (APS) and a high velocity oxy fuel process (CDS) have been employed to produce hydroxyapatite coatings. An X-ray diffraction technique (XRD) has been applied to measure the residual stresses in thermal sprayed hydroxyapatite coatings. It has been shown that such stresses are sensitive to spraying parameters and that the newer high velocity oxy fuel spraying process results in lower residual stresses than the conventional air plasma spraying process. Heat treatment of the coating has been shown to significantly reduce the residual stress in the coating.

The effect of hydroxyapatite (HA) coating on cortical bone apposition around press-fit inserted implants and implants surrounded by a gap was investigated. Uncoated and HA-coated titanium implants were inserted in burr holes with three different diameters in the tibia of rabbits. Implantation time was four months. The histological evaluation demonstrated that after four months implantation the interfacial bone reaction appeared to be identical for HA-coated and non-coated implants with various degrees of surgical fit. Although after four months the interface showed the same response, there still might be an initial advantage of the HA-interface with bony tissue.

A quantitative evaluation, at the scanning electron microscopic (SEM) level, was made of the osteoconduction and osseointegration of Ti-6AI-4V implants with and without plasma-sprayed hydroxyapatite coatings (HACs). By employing the Chinese Coin implant model in the lateral femora cortices of canines, different biological properties between HA-coated and uncoated Ti-6AI-4V implants could be compared in one specimen. After 4, 6 and 12 weeks, the implants with surrounding bone were removed and assessed histologically in undecalcified sections under SEM. The osteoconductivity and the ability of osseointegration of implants were histomorphometrically analysed from back-scattered electron images (BEIs) and represented in terms of the new bone healing index (NBHI) and apposition index (AI), respectively. Throughout all implant periods, the HA-coated Ti-6AI-4V implants revealed higher NBHI than the uncoated ones, it appearing that the HA-coated Ti-6AI-4V implants revealed more osteoconductive than the Ti-6AI-4V was. For HA-coated implant, the evidence of direct bone-to-HAC contact was observed. However, at the bone/Ti-6AI-4V interface, there intervened a fibrous membrane without calcium content, indicating that the Ti-6AI-4V implant was not osseointegrated in the SEM field of view. The maximum value of AI was reached 6 weeks after implantation for HA-coated implant, implying that the HAC had a stimulating influence on bone apposition within 6 weeks of healing. The signs of partial dissolution of HACs within the remodelling canals were evident at the HAC-bone interface 12 weeks after implantation, accounting for the slight decrease in NBHI and the obvious decrease in AI for HAC implant.

A new polymeric route using calcium nitrate and phenyldichlorophosphine as initial compounds and acetone as solvent is described. Hydrolysis and subsequent oxidation with air led to the formation of a viscous solution which could be transformed to a solid polymer by subsequent drying. Calcining this polymer at temperatures in the range of 900 to 1100°C led to the formation of hydroxyapatite possessing a slightly distorted lattice. Dip-coating of alumina and titanium substrates using highly viscous solutions, or repeatedly carried out dipcoating, led to the formation of thick, highly porous hydroxyapatite layers.