P. M. Marquis

Microstructural characterization of hydroxyapatite coating on titanium

The microstructure of hydroxyapatite plasma sprayed onto titanium alloy has been studied by using transmission electron microscopy. It has been shown that while substantial portions of the coating are crystalline hydroxyapatite, regions of amorphous calcium phosphate with Ca/P ratios of 0.6–1.0 are also present, both in the coatings and at the metal-ceramic interface. The microstructures observed have also been found to be consistent with devitrification of the amorphous calcium phosphates producing regions of very fine grained hydroxyapatite. A calcium titanate phase has also been detected at the metal-ceramic interface produced by the chemical reaction of hydroxyapatite to titanium.

Effect of heat treatment on the microstructure of plasma-sprayed hydroxyapatite coating

One of the claimed benefits of plasma-sprayed hydroxyapatite coatings on metal prostheses is the generation of enhanced bone bonding. However, plasma-sprayed hydroxyapatite undergoes a range of transformations during spraying, and the final microstructures produced are complex, with a variety of phases present. The microstructures of plasma-sprayed hydroxyapatite coatings on titanium alloy substrate which have been subjected to post-heat treatment at 950 degrees C have been characterized using transmission electron microscopy. It was demonstrated that heat treatment can modify the coating microstructure and improve adhesion between coating and substrate. The results of chemical analysis revealed an increase in the Ca:P ratios, accompanied by the transformation of calcium phosphate phases from amorphous to crystalline. These transformations are related to phosphorus diffusion into the titanium alloy substrate, which results in the formation of a Ti3P phase.

Selective laser sintering of ultra high molecular weight polyethylene for clinical applications

Rapid prototyping is a relatively new technology, which although prominent in the engineering industry is only just starting to make an impact in the medical field. Its current medical uses are mainly confined to surgical planning and teaching, but the technology also has the potential to allow for patient-tailored prostheses. The work reported here describes the application of a simplified selective laser sintering apparatus with ultra high molecular weight polyethylene (UHMWPE). The morphology and chemistry of the starting powders and lased material have been characterized using Fourier Transform Infra Red spectroscopy and a combination of light and scanning electron microscopy. It was found that solid linear continuous bodies could be formed, but material shrinkage caused problems when trying to form sheet-like structures. The porosity of the formed material was also a concern. The material exposed to the laser beam was shown to have undergone degradation in terms of chain scission, cross-linking, and oxidation. It has been concluded that to apply this technology to the fabrication of UHMWPE devices requires the development of improved starting powders, in particular with increased density.

Dissolution behavior of plasma-sprayed hydroxyapatite coatings

The long-term stability of plasma-sprayed hydroxyapatite coatings is influenced by the dissolution behavior of the coating in in vivo conditions. Plasma-spraying generates a mixture of phases and this study has focused on how the balance of phases affects the in vitro dissolution behavior of the coatings in double distilled-deionized water and in tris-buffer solutions. The pH changes in double distilled-deionized water were monitored, whilst the pH value was maintained at 7.25 for the tris-buffer solution at 37 °C with 5% CO2atmosphere. The phosphate and calcium ions released were measured using UV-Visible Spectrophotometer and Atomic Absorption Spectroscopy respectively. Changes in crystal and surface topology were also studied. The results indicate that the dissolution behavior of the coatings depends on several factors. The rate of release of phosphate ions was found to increase significantly for the tris-buffer solution compared to the deionized water, indicating that the presence of electrolyte constituents affects the dissolution behavior of the coatings. The Ca/P ratio in the tris-buffer solution is approximately three. Increases in the level of crystallinity of the coatings significantly decreased the dissolution rate and hence, the amount of phosphate ions released. The higher the percentage of crystallinity, the higher the stability of the coating under in vitro conditions.

An improvement in processing of hydroxyapatite ceramics

Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in “softening” particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min−1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.

Effect of pH on protein adsorption to hydroxyapatite and tricalcium phosphate ceramics

Calcium phosphate ceramics used for clinical applications vary considerably in their phase composition. A range of hydroxyapatite and tricalcium phosphate powders, some of which are used in plasma-sprayed coatings for metal prostheses, were found to leach substances which change the pH of water and cell culture medium. The adsorption of serum proteins from cell culture medium to the same hydroxyapatite and tricalcium phosphate ceramics was compared by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and individual bands identified by enhanced chemiluminescence immunodetection and gas chromatography sequencing. The iron transport protein transferrin showed distinct differences in adsorption to a variety of hydroxyapatite and tricalcium phosphate powders, with higher concentrations of transferrin adsorbed to powders which caused no change or a drop in the pH of cell culture medium or water. Transferrin was also adsorbed to Biotal HA 120 in greater concentrations at a lower pH than it was at a higher pH. These preliminary studies show that pH changes brought about by the leaching of soluble ions may have an effect on protein adsorption.

Monomer conversion versus flexure strength of a novel dental composite

OBJECTIVES To quantify the monomer conversion and flexural strength of an experimental oxirane-based composite material (EXL596) compared with two, commercially available, dimethacrylate based restoratives (Z250 and Z100). METHODS Fourier-transform infra-red spectroscopy (FTIR) was utilised to evaluate the degree of conversion (DC) (n=5) and biaxial flexure strength (BFS) testing (n=20) was used to analyse flexural strength and associated Weibull moduli (m) of each material following 0.1, 0.5, 1, 4, 24 and 48 h immersion in a lightproof waterbath maintained at 37+/-1 degrees C. RESULTS The DC of Z250 and Z100 following 0.1, 0.5 and 1 h post-irradiation was significantly greater than the DC of EXL596 for the same immersion periods. This was manifested as a significant decrease in BFS and associated m of EXL596 compared with Z250 and Z100 for the 0.1, 0.5 and 1 h post-irradiation periods. The DC and BFS of EXL596 were significantly greater than Z250 and Z100 following 24 h immersion. CONCLUSIONS Assessment of FTIR spectra, BFS and associated m has provided a useful method in the quantitative analysis of resin-based composite conversion. Identification of the decreased DC of EXL596 compared with Z250 and Z100 was achieved using FTIR. However, decreased conversion rates within the first hour following irradiation of EXL596 may compromise flexural strength properties (associated with a decrease in BFS and m) which may be inadequate under masticatory loading.

The Strengthening Mechanism of Resin Cements on Porcelain Surfaces

All-ceramic crowns bonded with resin cements have increased performance, and two theories have been proposed. Marquis (1992) suggested that the resin modified defects by crack healing, while Nathanson (1993) proposed that resin polymerization shrinkage strengthened porcelains. Both theories imply a sensitivity of strengthening to defect size. The hypothesis tested was that resin strength enhancement is independent of defect severity. We ground 200 porcelain discs to remove imperfections and indented 120 to create a large defect. Discs were tested dry, wet, and after being coated with 75–100 μm of resin cement in bi-axial flexure. Disc strength with and without indentations was increased significantly when coated with 2 resin cements. Both cements significantly increased the strength independent of defect population, and the hypothesis was accepted. It is proposed that the combination of surface pre-treatment and cement moved the fracture origin from the porcelain/cement interface to the cement surface, consistent with resin strength enhancement independent of defect severity.

Potential countersample materials for in vitro simulation wear testing

OBJECTIVES Any laboratory investigation of the wear resistance of dental materials needs to consider oral conditions so that in vitro wear results can be correlated with in vivo findings. The choice of the countersample is a critical factor in establishing the pattern of tribological wear and in achieving an efficient in vitro wear testing system. This research investigated the wear behavior and surface characteristics associated with three candidate countersample materials used for in vitro wear testing in order to identify a possible suitable substitute for human dental enamel. METHODS Three candidate materials, stainless steel, steatite and dental porcelain were evaluated and compared to human enamel. A variety of factors including hardness, wear surface evolution and frictional coefficients were considered, relative to the tribology of the in vivo situation. RESULTS The results suggested that the dental porcelain investigated bore the closest similarity to human enamel of the materials investigated. SIGNIFICANCE Assessment of potential countersample materials should be based on the essential tribological simulation supported by investigations of mechanical, chemical and structural properties. The selected dental porcelain had the best simulating ability among the three selected countersample materials and this class of material may be considered as a possible countersample material for in vitro wear test purposes. Further studies are required, employing a wider range of dental ceramics, in order to optimise the choice of countersample material for standardized in vitro wear testing.

Resin Elasticity and the Strengthening of All-ceramic Restorations

Resin luting of all-ceramic restorations results in increased performance; however, the strengthening mechanism and the role of the mechanical properties of the resin are not fully understood. The hypothesis tested is that ceramic strength enhancement is dependent on the elastic modulus of the resin. Three-point flexural moduli of a flowable, luting, and hybrid composite resin were characterized. Two hundred forty porcelain discs were air-abraded. One group acted as a control, and 3 additional groups were coated with 120 ± 20 μm of each resin prior to bi-axial flexure testing. All resins significantly increased in mean strength, and the associated strength increase was related to the elastic modulus of the resin (R2 = 0.9885), so the hypothesis was accepted. The combination of Poisson constraint and the creation of a resin-inter-penetrating layer sensitive to the elastic modulus of the resin may provide an explanation of the strengthening mechanism.