Jean-Marc Meyer

Comparison of the color stability of ten new-generation composites: an in vitro study.

OBJECTIVES The aim of this in vitro study was to evaluate the color stability of modern light-cured composites when subjected to various physico-chemical and staining conditions. METHODS Ten brands were evaluated including hybrids, microfine hybrids and microfilled composites. Some universal shade samples underwent only staining tests, while others were subjected to one of the following experimental conditions: thermocycling, postcuring, polishing or a 1 wk immersion in saline, prior to staining. The coloring solutions used for the staining tests were: coffee, E 110 food dye, vinegar and erythrosin. A colorimetric evaluation according to the CIE L*a*b* system was performed after experimental periods of 1 and 3 wk. RESULTS Erythrosin caused the greatest color change for the composites tested. A reduced susceptibility to staining was observed where surfaces had been polished. Low water sorption, a high filler-resin ratio, reduced particle size and hardness, and an optimal filler-matrix coupling system were related to improved composite resistance to discoloration. SIGNIFICANCE Resistance of modern composites to discolorations still depends on their structure and manipulation.

Bioadhesive intraoral release systems: design, testing and analysis

The aim of this study was to analyse the adhesion of various polymeric formulations and to measure the in vitro release of a local anaesthetic (febuverine hydrochloride) from a hydrophobic gel containing 34% (wt/wt) adhesive hydrocolloids. The bioadhesive polymer system was prepared from a polyethylene gel containing various amounts of sodium carboxymethylcellulose (NaCMC) as the adhesive, and hydrolysed gelatin as the water sensitive material to ensure rapid swelling. Upon hydration, adhesion studies were performed with a tensile tester equipped with a custom-made cell. The controlled release of the active ingredient was studied with a dissolution cell filled with artificial saliva at 37 degrees C. It was found that the relative adhesive bond strength of the formulations was dependent on the NaCMC content, showing a maximum at about 20 wt %. Febuverine hydrochloride release achieved an optimal release rate for formulations with NaCMC in the range of 12-25 wt %.

Effects of fluorides on titanium and other dental alloys in dentistry.

Fluoride ions are the only aggressive ions for the protective oxide layer of titanium and titanium alloys. Thus their presence may possibly start a localized corrosive degradation by pitting and crevice corrosion processes. Since hygiene products like toothpastes and prophylactic gels contain fluoride ions, the present study has been completed to evaluate the effect of fluoride ions on titanium and dental alloys used, for example, as dental implants and superstructures. Two different milieu based on the Fusayama artificial saliva and an electrolyte solution containing NaCl, with and without fluoride ions, have been used for the electrochemical tests, in a pH range of 6.15 to 3.0. Open circuit potentials, potentiodynamic curves, Tafel slopes, galvanic couplings and crevice potentials are the electrochemical procedures selected for this work. Based on the experimental results obtained with these procedures, the mixed potential theory was applied to predict couple potentials and couple currents. It has thus been shown that: (a) with and without fluoride ions, galvanic currents are weak (10E - 7 to 10E - 8 A cm(-2)) within a pH range of 6.15 to 3.5; (b) titanium submitted to anodic polarization in an electrolyte, even one containing fluoride, merely develops an oxide layer and does not corrode within that same pH range of 6.15 to 3.5; (c) in confined areas where fluoride ions are present, titanium and the dental alloys tested undergo as corrosive process, in the form of crevice and pitting, as soon as the pH drops below 3.5.

Mechanical behavior of glass ionomer cements affected by long-term storage in water

OBJECTIVES The early strength of glass ionomer cements was investigated previously (Cattani et al., 1993). Using the same set of glass ionomer cements, this study was designed to determine if mechanical property degradation occurred after an extended period of storage in water. METHODS In the present study, the effect of aging in water on the compressive, tensile and flexural strengths of twelve commercial glass ionomer cements was evaluated at different time intervals after the beginning of the setting reaction. The aging periods varied from 24 h to 12 mon. RESULTS The results show that the mechanical properties of the tested materials are in some cases affected by long-term aging in water. The evolution of strength follows distinct patterns of change. It has been established that the glass ionomer cements are neither characterized by a continuous increase nor a continuous decrease in strength. Most of the materials tested maintained a constant strength. However, several of them first display an improvement of the mechanical strength followed by a reduction. In this case, the 24h and the 12 mon strengths are usually similar. SIGNIFICANCE The aging mechanisms of glass ionomers are complex. Strengthening probably results from additional crosslinking and build-up of a silica gel phase, whereas weakening may result from erosion and plasticizing effect of water.

Comparative study of the physical properties of a polyacid-modified composite resin and a resin-modified glass ionomer cement

OBJECTIVES The physical properties of the resin-modified composite resin Dyract (Detrey Dentsply) and the resin-modified glass ionomer cement Fuji II LC (GC) were compared, and the effect of water sorption on these properties was studied. METHODS Compressive, diametral compressive and flexural strengths were measured using specimens aged up to three months. The Vickers hardness and the water erosion were also determined. The specimens were stored at 37 degrees C under five different conditions, chosen to vary the water sorption of the samples. The results were analyzed using a multi-factor analysis of variance. RESULTS Dry specimens of Dyract and Fuji II LC showed similar properties. However, the two materials behaved differently when stored in presence of water. In contrast to Fuji II LC, Dyract showed very little expansion for the first 24 h (1.5 +/- 0.3 and 0.03 +/- 0.01%, respectively), leached small quantities of ions and retained its mechanical strength. Those differences are related to their chemical composition. Fuji II LC is hydrophilic, as it contains polyHEMA. In the presence of water, Fuji II LC behaves like a hydrogel, but the network resulting from the copolymerization of acidic and UDMA monomers is less hydrophilic, and the effect of water on Dyract is retarded. SIGNIFICANCE The significant properties of Dyract are determined by its composite character. This certainly represents some advantages, for instance, a higher mechanical strength, a better protection against initial dehydration and subsequent water effects. However, Dyract shows some disadvantages over Fuji II LC, like a lower amount of fluoride release or the interference of oxygen during polymerization.

Effect of cement film thickness on the fracture resistance of a machinable glass-ceramic

OBJECTIVES The aim of this study was to determine the fracture resistance of a machinable glass-ceramic plate cemented to a resin composite block as a function of the cement film thickness for two types of cement. METHODS Ceramic plates were cemented to resin composite blocks using either zinc phosphate cement or a resin composite cement. For the zinc phosphate cement, the film thickness was 33 +/- 8 microns or 128 +/- 8 microns; for the resin composite cement, the thickness ranged from 26 +/- 11 microns to 297 +/- 48 microns. The elastic modulus was determined for each of the cements. Fracture loads were obtained by using a spherical steel indenter in the center of the glass-ceramic plate. The Weibull distribution was used for the statistical analysis. RESULTS For glass-ceramic plates cemented with zinc phosphate cement, the fracture resistance was independent of the film thickness. When the resin composite cement was used, a gradual decrease of the fracture strength was observed that became statistically significant at a cement thickness of 300 microns or more. The characteristic fracture strength of glass-ceramic plates cemented with the resin composite cement was about 75% higher than when using the zinc phosphate cement. This difference is attributed to the bonding of the resin cement to the ceramic plate and the supporting structure. SIGNIFICANCE The findings of this study suggest that the resistance to fracture due to indentation of the glass-ceramic may not be affected by the cement film thickness as much as previously thought.

Corrosion behavior of a welded stainless-steel orthopedic implant

The corrosion behavior of combinations of materials used in an orthopedic implant: the spherical part (forged or forged and annealed) constituting the head, the weld (tungsten inert gas (TIG) or electron beam (EB) techniques), and the cylindrical part (annealed) constituting the shaft of a femoral prosthesis - has been investigated. Open-circuit potentials, potentiodynamic curves, Tafel slope, mixed potential theory and susceptibility to intergranular attack are electrochemical and chemical procedures selected for this work. Electrochemical measurements using a microelectrode have been made in the following zones: spherical part, cylindrical part, weld, and weld/sphere, and weld/shaft interfaces. To detect intergranular attack, the Strauss test has been used. At the interfaces, corrosion currents, measured (Icorr) and predicted (Icouple) are low, in the order of the pico- to nanoampere. The electrochemical behavior of the electron beam (EB) weld is better than that of the tungsten inert gas (TIG). Welds at interfaces can behave either anodically or cathodically. It is better if welds, which are sensitive parts of the femoral prosthesis, behave cathodically. In this way, the risk of starting localized corrosion (pitting, crevice or intergranular corrosion) from a galvanic couple, remains low. From this point of view, the sample with the EB weld offers the best behavior. All the other samples containing a TIG type of weld exhibit a less favorable behavior. The mechanical treatments (forged, and forged and annealed) of the steel sphere did not show any difference in the corrosion behavior. No intergranular corrosion has been observed at the weld/steel interface for unsensitized samples. With sensitized samples, however, a TIG sample has exhibited some localized intergranular corrosion at a distance of 500 microm along the weld/stainless steel (sphere) interface.

Pitting, crevice and galvanic corrosion of REX stainless-steel/CoCr orthopedic implant material.

The corrosion behavior of surgical implant CoCr alloy and REX 734 steel has been investigated. The pitting or crevice corrosion potentials have been determined to reach values as high as 500 mV vs. SCE for CoCr and 450 mV vs. SCE for REX 734. The galvanic corrosion behavior of CoCr/REX 734 couples has been evaluated with various electrochemical techniques. The measurement of the corrosion current of the galvanic couple as well as its prediction by applying mixed potential theories on measured potentiodynamic polarization curves revealed low galvanic currents in the range of nanoamperes.

Early strength of glass ionomer cements

The present study compared the compressive, tensile and flexural strengths of 26 commercial glass ionomer cements, which were evaluated 24 h after the beginning of the setting reaction. In order to give a global estimation of their overall strength, a coefficient was attributed to each glass ionomer cement. This strength coefficient provided a ranking of the tested materials. The highest scores were obtained by the restorative glass ionomers; however, these materials exhibited lower mechanical properties than the more classic restorative materials such as amalgams or composites. The results also showed a large scattering of the mechanical properties among the luting and lining glass ionomer cements. The creep behavior of the glass ionomer cements was also investigated.

Corrosion resistance of nickel-chromium dental casting alloys

Abstract The corrosion resistance of twelve Ni Cr dental casting alloys has been assessed by registrating their potentiodynamic polarization curves, polarization resistance curves, and potential-time curves, in a de-aerated and aerated artificial saliva. It has been found that not only the presence of chromium is needed to reduce the electrochemical activity of such alloys, but also that of molybdenum and manganese. The tendency to passivation is enhanced by these two elements, along with the resistance to pitting in a chloride-containing electrolyte. Alloys with low Cr content and without Mo did always corrode in the tested conditions.