Yannick Fovet

About a synthetic saliva for in vitro studies

Numerous artificial salivas have been used during studies in odontology. These salivas have compositions, which are more or less the same as that of natural saliva. In this article, we are presenting a discussion about the various media described in the related literature. A review of nearly 60 artificial salivas was carried out to clarify the role of some of the compounds most frequently met in the proposed formulae. The study focused on the buffer effect, the role played by CO(2) gas and the presence of calcium ions, hydrogenocarbonates, hydrogenophosphates and thiocyanates. The SAGF medium, which we proposed some years ago, was used as a reference and some in vitro behavioral tests of dental biomaterials were studied in a comparative way. Using the SAGF medium allowed us to specify the mode of fluoride ions release from glass ionomer cements and the corrosion behavior of the dental amalgams.

In vitro fluoride release from restorative materials in water versus artificial saliva medium (SAGF)

OBJECTIVES An extensive number of investigations have focused in recent years on the process of fluoride release by glass-ionomer cements. In order to compare the fluoride release of two recent resin-modified glass-ionomer cements (RMGICs) and one compomer, we investigated the initial and subsequent fluoride release in distilled water and in a mineral medium with composition similar to saliva (SAGF), as well as the renewal effect of the media on the release. METHOD A preliminary thermodynamic study defined conditions under which the measurements by fluoride-selective electrode potentiometry were not affected by the presence of Ca2+ ions in SAGF. Disk-shaped samples (5 mm x 3 mm) for each material were placed in 5 ml of distilled water (n = 12) or 5 ml of SAGF (n = 12) and fluoride released was measured after 1, 2 and 7 days. For the investigation of medium renewal effect, RMGIC sample were dipped into distilled water for 5 days. Subsequently 12 samples of each material were placed for 7 days into water or SAGF, with or without daily renewal of the medium before fluoride-release measurements. The elements present on the surface of the RMGICs were analyzed by scanning electron microscopy. RESULTS RMGICs released significantly more fluoride in water than in artificial saliva. The presence of CaF2 at the interface accounts for this phenomenon. The thickness of the CaF2 layer depends on the speed of its initial formation and renewal of the testing medium. A pattern of CaF2 formation was proposed.

Mechanisms of scale formation and carbon dioxide partial pressure influence. Part I. Elaboration of an experimental method and a scaling model

Scale formation in industrial or domestic installations is still an important economic problem. The existence of a metastable domain for calcium carbonate supersaturated solutions and its breakdown are observed under conditions rarely well defined. In most cases it is the pH rise caused by the carbon dioxide loss that involves calcium carbonate precipitation. Before studying this problem, we suggest in this first part, a new model for the evolution of the calcocarbonic system that takes into account the hydrated forms of CaCO3: CaCO3 amorphous, CaCO3 x 6H2O (ikaite) and CaCO3 x H2O (monohydrate). According to this model, the precipitation of any one of these hydrated forms could be responsible for the breakdown of the metastable state. After this first step, the solids evolve into dehydrated forms. At first, the metastable domain spread of the calcium carbonate supersaturated solutions was studied by the elaboration of computer programs in which the formation of CaCO3(0)(aq) ion pairs was taken into account. These ion pairs are supposed to evolve through dehydration to form the various calcium carbonate solid form precursors. This thermodynamic study was then compared to the experimental methods of the critical pH. Here the pH rise was caused by adding sodium hydroxide under different conditions for sodium hydroxide addition speed, agitation mode and ageing of solutions. For the highest speed of sodium hydroxide addition, the CaCO3 ionic product reached the value of the amorphous calcium carbonate solubility product, and the reaction of the amorphous calcium carbonate precipitation was of the homogenous type. Decreasing the reagents addition speed caused an extension of the titration time. Then, the breakdown of the metastable state was obtained with the CaCO3 x H2O heterogeneous precipitation. This clearly illustrates the probable ageing of the precursors of the solid states that are considered in this model.

Corrosion by galvanic coupling between carbon fiber posts and different alloys

OBJECTIVES The corrosion of metal type (NiCr, copper, etc.) posts in post and core restorations has been a well known fact over the last few years. A new generation of carbon fiber posts is now playing an extensive role in dental prosthetics, but these posts may, in some cases come in contact with alloys used for restoration purposes. The objective of this paper is to study the electrochemical behaviour of these posts in an artificial SAGF medium (Artificial Saliva Gal Fovet) for comparison with that observed for different alloys (gold, NiCr and amalgams) in the same medium, and to assess the coupling that might occur between these posts and alloys. METHODS Posts of different diameters (1-2.1 mm) were molded in resin, thus forming 10 x 3 mm pads that were mounted on a rotating electrode. Pads of identical dimensions were also made for each alloy. Voltammograms (current-tension curves) were plotted for every material in the SAGF medium at 25 degrees C with normal aeration and low aeration (bubbling purified argon) using a potentiometer with three electrodes to determine the position of the zero current potentials. Evans graphs were then plotted once the anode part of the alloy curves and the cathode part of the post curves had been processed with computer software. The potentials and the maximum coupling current densities were estimated. RESULTS Posts may be involved in galvanic coupling in the presence of oxygen. Galvanic activity in an aerated solution is highly probable upon contact with amalgams, but may also occur if the material in contact is NiCr alloy. Coupling between precious alloys and posts is improbable and can only induce very low currents in the presence of oxygen. SIGNIFICANCE This study devoted to coupling of carbon fiber posts and metals in the mouth, attracts attention to the fact that a corrosion reaction can be set-up in the infiltration conditions relevant to the prosthetic restoration.

Formation constants β2 of calcium and magnesium fluorides at 25°C

Abstract Until now, scientific work on the reactions of the complexation in water of fluoride ions with calcium or magnesium ions considered only soluble order 1 complexes (CaF+ and MgF+). The precipitation of the compounds CaF2 and MgF2 did not take into account the preliminary formation in solution of these order 2 complexes (CaF20 and MgF20). We therefore studied their formation using a direct potentiometric method in the F− ion-selective electrode at 25°C. This experiment, together with a computerized thermodynamic study, allowed us to determine the values of formation constants at ionic strength equal to zero: log β CaF 2 0 =5.7 and log β CaF 2 0 =3.2 , as well as limiting solubilities: SCaF2=10−4.8 and SMgF2=10−5 mol l−1. Then, some examples were studied showing the necessity to take into account the formation constant of CaF2: the supersaturation of solutions, the validity of results from the fluoride selective electrode, the difference between released fluoride in distilled water and in artificial saliva in restorative dentistry, the fluoride concentration in hard waters and its health consequences.

Mechanisms of scale formation and carbon dioxide partial pressure influence. Part II. Application in the study of mineral waters of reference.

In the first part, we have designed a new model of evolution for the calco-carbonic system which includes the hydrated forms of CaCO3: CaCO3 amorphous, CaCO3 x 6H2O (ikaite) and CaCO3 x H2O (monohydrate) (J. Eur. Hydr. 30 (1999) 47). According to this model, it is the precipitation of one or other of these hydrated forms which could be responsible for the breakdown of the metastable state. After this first step, the precipitates evolve to dehydrated solid forms. Through the elaboration of computer programs in which the CaCO3(0) (aq) ion pair formation was considered, this model was compared to experimental data obtained by the critical pH method applied to synthetic solutions. In the present article, the same method was applied for four French mineral waters, at 25 degrees C under study. Three samples formed a precipitation during the sodium hydroxide addition. For these three cases, this precipitation began for the CaCO3 H2O saturation. The added volume of sodium hydroxide was more than what was required for neutralizing free CO2 initially in solution. These results indicate that during a spontaneous scaling phenomenon, the pH rises at the same time by loss of the initial free CO2 and of the one produced by the hydrogen carbonate ions decomposition. Then we calculated, at various temperatures for the three studied scaling waters: CO2 partial pressures and loss of total carbon corresponding to the solubility products of CaCO3 hydrated forms. The results show that the partial pressure monitoring of the carbon dioxide is important in managing the behavior of scaling waters.

Repartition of various soluble and insoluble species in supersaturated solutions

The hydrated ion-pair model for uncharged soluble species in equilibrium with many solid polymorphs is applied to examples fiom various fields of interest. The role of such precursors and their changes on ageing can explain the often observed evolution of a hydrated precipitate to successively more dehydrated solid forms. The cases studied concern the precipitation of calcium carbonates, oxalates and phosphates.