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Biomedical subjects

A D Wilson

Publications and source records attributed to A D Wilson.

208 records · Page 12Linked to original sources
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Zinc polycarboxylate cements: a chemical study of erosion and its relationship to molecular structure.

Experimental results show that the rate of elution of Zn2+ and Mg2+ ions from polycarboxylate cements decrease with the cement's age. This phenomenon is explained by the setting reaction in which these cations are liberated from the oxide powder by an exchange with protons from the polyacid and in this state are vulnerable to water leaching. Subsequently, these cations become bound to the polyanion chain and become more resistant to water leaching. Magnesium ions are more easily hydrolyzed than zinc ions because they are less strongly bound to the polyanion chain. Zinc ions are apparently more strongly bound to a polyacid which is a copolymer of acrylic and itaconic acids, than to the homopolymer of acrylic acid. The pattern of elution of ions differs between the different cements, so that measurements after 24 hours cannot be used to predict long-term durability on a comparative basis. This coupled with the slow but continuous loss of ions makes a 24-hour test of doubtful validity. Polycarboxylate cements absorb water that is present in the matrix in loosely bound and tightly bound forms. The ionization of the polyacid to a polyanion during the course of the reaction apparently creates a demand for water, and the cement where the COOH:C ratio in the polyacid is highest absorbs the most water.

Absorption↗
PubMedDate not supplied

Reactions in glass-ionomer cements: IV. Effect of chelating comonomers on setting behavior.

The oscillating rheometer is a valuable instrument for studying the effects of additives on the setting behavior of a cement system. Using this instrument, it was found that certain chelating comonomers, the hydroxycarboxylic acids, could improve the setting characteristics of the glass-ionomer cement system when added to the PAA solution. The acid chelates probably assign the extraction of metal ions from the glass and also tend to hold them in solution, preventing premature ion binding of the polyanion chains. The effect is to increase the rate of hardening without reducing the working time, which may indeed by slightly increased. Tartaric acid, the most effective of the comonomers, can form a chelate bridge between aluminum atoms, and this metal complex probably acts as a flexible bridge structure linking polyanion chains. This mechanism offers some steric advantages over a simple salt bridge.

Acrylic Resins↗
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Specification test for the solubility and disintegration of dental cements: a critical evaluation of its meaning.

The specification test for solubility and disintegration is an essential screening test for the quality control of certain cement types. No cement fails it can be considered for use; however, the test has only a limited clinical significance because it does not give an indication of the stability of the fully hardened cement matrix either in oral fluids or water. Instead, it gives a measure of the content of the soluble reaction intermediate present in cements that are still hardening and where the matrix has not fully formed. The test relates, therefore, to early vulnerability to aqueous attack and hardening rate. The period of test is, in fact, an atypical one and the true extent of the erosion of the cement matrix is obscured by the presence of soluble reaction intermediates. Long-term extrapolations cannot be made and so test results for broadly satisfactory cements within a category cannot be used for valid comparisons of clinical excellence, and they can be used even less to compare different cement types with different setting reactions. For example, in general, the dental silicate cement has a higher solubility and distinegration figure than the zinc phosphate cement because it contains soluble sodium salt and hardens more slowly. However, it is more stable under oral conditiont an indication of the vulnerability of the forming matrix to attack. The test is not valid when applied to zinc oxide-eugenol cements because the products of decomposition are either virtually water-insoluble or volatile and therefore not measured under the conditions of the test. A method is suggested for overcoming this deficiency. The test needs to be supplemented by other tests done on fully hardened cements for longer periods of time and in mediums that can represent oral conditions. One example would be to stimulate acid conditions developed in stagnation areas. A total estimation of decomposition products, both soluble and insoluble, is called for rather than the determination of soluble materials only.

Absorption↗
PubMedDate not supplied

Reactions in glass ionomer cements: V. Effect of incorporating tartaric acid in the cement liquid.

A description is give of the effect on the ASPA cement reaction of tartaric acid incorporated in the cement liquid. Tartaric acid acts as an accelerator that aids in the extraction of ions from the aluminosilicate glass and facilitates their binding to the polyanion chains. Postgelation hardening is significantly increased. Working time is unaffected possibly because cations are initially present as complexes.

Acrylates↗
PubMedDate not supplied

Glass ionomer cements: chemistry of erosion.

A three-month study of the chemistry of the water erosion of two forms of ASPA cement has been made. The effect of varying cement consistency and cure time was investigated. The results are discussed in terms of the known chemistry and structure of the cement. The erosion behavior is compared to that of silicate, silicophosphate, and zinc polycarboxylate dental cements. The state of absorbed water and the mechanism of erosion is discussed.

Acrylates↗
PubMedDate not supplied

Balloon dilatation for discrete subaortic stenosis: immediate and intermediate-term results.

Six children with subvalvar aortic stenosis underwent percutaneous balloon angioplasty over a 15-month-period ending October 1989. The mean systolic pressure gradient across the left ventricular outflow tract decreased from 56 +/- 19 (mean +/- SD) to 12 +/- 7 mmHg (p less than 0.001) immediately following valvuloplasty and the degree of aortic insufficiency did not significantly increase. Follow-up Doppler data (in all 6 patients) were available 3 to 16 months (mean, 11 months) after angioplasty and revealed a residual aortic subvalvar gradient of 21 +/- 5 mmHg, which continues to be significantly lower (p less than 0.001) than that prior to angioplasty. There was no increase in aortic insufficiency. The single infant with increase in gradient at followup was determined to have fibromuscular, tunnel type of subaortic obstruction. None of the five patients with discrete membranous obstruction had significantly increased their gradients. Use of balloons larger than aortic valve anulus did not produce any adverse effect, particularly aortic insufficiency. We surmise that the immediate and intermediate-term follow-up results of balloon angioplasty are encouraging and balloon angioplasty should be considered as a treatment option in the initial management of discrete subaortic membranous stenosis.

Adolescent↗
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Dr. James Cronin Egan.

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History, Modern 1601-↗
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Resin-modified glass-ionomer cements.

The recently developed dual-setting resin glass-ionomer cements are discussed, and the question of whether they are the true glass-ionomer cements is addressed. Their chemistry and properties are examined and compared with conventional glass-ionomer cements. Resin-modified glass-ionomer cements have the advantage of a long working time combined with a rapid set and higher early strength. They are also easily bonded to resins. They have the strength properties comparable to conventional glass-ionomer cements rather than composite resins, but they share with composite resins the disadvantage of containing free monomers and therefore may not be as biocompatible as conventional glass-ionomer cements.

Glass Ionomer Cements↗
PubMedDate not supplied

Developments in glass-ionomer cements.

The performance of glass-ionomer cements has been improved since their introduction, and many of the initial disadvantages have been overcome. This article reviews the development of glass-ionomer cements, the problems involved, their solution, and their relation to underlying scientific principles. It describes improvements in mixing and setting characteristics, aesthetics, strength, and adhesion to tooth material. New bonding agents and resin-modified glass-ionomer cements are described.

Dental Bonding↗