The dissolution mechanisms of silicate and glass-ionomer dental cements.

The mechanism of dissolution of two dental cements of the acid-base setting types (silicate and glass-ionomer) is considered. Dissolution is incongruent, probably because most of the leached species can derive both from the matrix (polysalt gel) and the partly reacted glass particles. The release occurs by means of three discrete mechanisms, surface wash-off, diffusion through pores and cracks or diffusion through the bulk. Such behaviour is shown to be capable of being modelled with extremely high goodness-of-fit values, using equations such as y = const + at1/2 + bt. Analogies with research from the fields of geochemistry and nuclear fuel storage are made and these systems obey similar relationships. The dental cement systems differ, however, in that their dissolution is to some extent reversible. This is explained in terms of formation of insoluble complexes, either by reaction of the constituent ions, or by replacement of OH-, for example, with F-.

The release of fluoride and other chemical species from a glass-ionomer cement.

The elution of fluoride, sodium and silica from a glass-ionomer cement was studied for 598 days. It was found that these species were still being released when the experiments were concluded, however, the rate of release was much diminished. The release of fluoride, sodium and silica was incongruent. Only fluoride associated with sodium appeared to be available for release.

The morphological characterization of dental amalgam alloy powders.

Two commercially available amalgam alloy powders (one lathe-cut, the other spherical) were characterized in terms of density, surface area, particle size distribution and appearance under the scanning electron microscope. A third, experimental alloy powder, prepared by electrodeposition, was subjected to a similar analysis. It was found that particle size distribution can be a misleading indicator, in that when particles are porous, inordinately large amounts of mercury will be taken up on trituration, and the resulting amalgam will have wholly unsatisfactory mechanical properties. Surface area measured by BET (gas adsorption) may be the best single index of the morphological quality of an amalgam alloy powder.

An assessment of the jet-test method for solubility measurement of dental cements.

The use of the jet-test method for laboratory measurements of dental cement solubilities was explored. Two cements, a glass-ionomer and a silicate, were used as models. Lactic acid concentrations in the range 0.1-0.005 M, ionic strength, the effect of column height, the effect of continuous and intermittent flow, and the effect of temperature were investigated. Several modifications to the jet-test equipment were made as a result of these studies. All the above-mentioned variables were shown to be critical in obtaining reproducibility, and a failure to control them will most probably give invalid results. The mode of wear (which is linear with time in most cases) is shown to include a significant element of erosive wear as well as one based on chemical attack.

A model for the dissolution and fluoride release from dental cements.

Data on the rate of dissolution (including fluoride ion release) of silicate and glass ionomer cement samples of variable surface area:volume ratios are examined in conjunction with dye penetration experiments and a simultaneous EDX analysis of the five elements F, Si, P, Al, Ca., as obtained by a series of point analyses taken at 10 micron intervals over a length of 7mm on a cement sample exposed to two months continuous immersion. Adding to this unpublished information on the surface area (from BET) and porosity (mercury porosimetry) it is shown that, in their behaviour, these cements behave as controlled release systems of the porous granular monolith type and that application of homogeneous monolith equations is inappropriate.

Tensile and shear strength measurements on brazed "T" joints used in construction of intra- and intermaxillary hooks.

Silver-brazed "T" joints, representing the first stage in the construction of intermaxillary hooks were fabricated under rigorously controlled conditions. Their tensile and shear strengths were then measured using specially developed test jigs. It was found that the braze joint itself was stronger than the component wires (under tensile load) while in shear mode, fracture occurred in one of a number of well-defined ways. The effect of prolonged heating was investigated using microhardness measurements and recommendations are made for the optimum brazing conditions.

Diatomite as a source of cristobalite for dental investments a differential thermal analysis study.

Cristobalite can be obtained from diatomite by heat-treatment at relatively low temperatures and short times without the use of flux. The differential thermal analysis method used to examine the alpha leads to beta inversion was not as satisfactory as it might have been, and to determine the amount and crystallinity of cristobalite formed any such method should be supported by other techniques, in particular, X-ray diffraction. To determine the suitability or otherwise of cristobalite produced from diatomite for use in dental investments, dialatometric studies are indicated since the property of thermal expansion is of fundamental importance to a dental investment.

Dissolution rates of silicate cements.

The literature relating to the dissolution of the silicate cements is reviewed, including the methodology, intrinsic factors relating to composition and method of mixing, curing etc. and extrinsic factors, i.e. the medium, pH, ionic strength etc. to which the sample of cement is exposed. New experimental data is shown relating the dissolution rate to solution composition and ionic strength. It is seen that samples of silicate cement can take up phosphate from solution as well as releasing this species and a dissolution process based on a reversible equilibrium is advanced. Also considered are the effects of sample volume:area ratio and the effect of agitation.

Corrosion of Co-Cr alloys in aqueous environments.

The literature relating to the corrosion in aqueous media of the Co-Cr alloys is reviewed. Data are surveyed for their corrosion in sulphuric acid, alkalis and in hydrochloric acid and chloride media. In the latter category, the information is sub-divided into single corrosion, crevice and stress corrosion, fretting corrosion and data obtained from 'in vivo' experiments. It is clear that the very low current densities which result from 'in vitro' experiments present abnormal experimental problems and may not, in every case, actually represent the dissolution process.