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Biomedical subjects

J W Nicholson

Publications and source records attributed to J W Nicholson.

At least 55 records · Page 3Linked to original sources
PubMed1998

Studies on the setting of polyelectrolyte cements: part VI. The effect of halide salts on the mechanical properties and water balance of zinc polycarboxylate and glass-ionomer dental cements.

A study is reported in which a zinc polycarboxylate and a glass polyalkenoate dental cement, respectively, were prepared from aqueous solutions of NaCl, KCl, KBr and KI, all at 1 mol dm3 concentration, as well as from pure water. For the zinc polycarboxylate, setting as determined by oscillating rheometry was speeded up and water uptake was enhanced by the presence of the salts. Conversely, compressive strength at 24 h was unaffected. On the other hand, for the glass polyalkenoate, the setting reaction was slowed down, water uptake inhibited and compressive strength at 24 h reduced (from 94.3 MPa with pure water to 59.8 MPa with NaCl, 65.8 MPa for KCl, 67.0 MPa for KBr and 81.1 MPa for KI). Previous work with polyelectrolytes in aqueous solution suggests that the halides probably enhance the rate of the neutralization process. For the zinc polycarboxylate, this leads to a more rapid setting reaction. By contrast, for the glass polyalkenoate, it results in slower setting and weaker cements. This result is attributed to inhibition of the secondary setting reaction, involving the formation of the silicate/phosphate network, by enhanced neutralization, a process which is consequently concluded to occur earlier in the overall setting of these cements than had been assumed previously.

Journal Article↗
PubMed1998

Studies in the setting of polyelectrolyte cements: part VII. The effect of divalent metal chlorides on the properties of zinc polycarboxylate and glass-ionomer dental cements.

A study is reported in which a zinc polycarboxylate and a glass polyalkenoate dental cement, were prepared from aqueous solutions of divalent metal chlorides, namely ZnCl2, CaCl2, MgCl2 and SrCl2, all at 1.0 mol dm-3 concentration, as well as from pure water. Calcium chloride was employed at additional concentrations, i.e. 2.0, 0.5 and 0.1 mol dm-3. As was previously found for monovalent salts, setting of the zinc polycarboxylate was speeded up and water uptake generally enhanced by the presence of the divalent metal salts. However, the divalent salts were found to reduce the compressive strength at 24 h (from 86 MPa to about 60 MPa). The glass polyalkenoate showed broadly similar effects to those observed in the presence of monovalent salts, with the setting time being increased, water uptake inhibited and compressive strength at 24 h reduced; however, by contrast, the working time was generally reduced. These results occur because the rate of the neutralization process is increased by the divalent salts, a consequence of the reduced pH of the poly(acrylic acid) caused by these salts. Infrared spectroscopy demonstrated interactions between the metal chlorides and poly(acrylic acid), with various chelate structures being apparent from the position of the asymmetric carbonyl stretch.

Journal Article↗
PubMed1998

The interaction of dental cements with aqueous solutions of varying pH.

A study is reported in which a series of dental cements of varying types (zinc phosphate, zinc polycarboxylate, glass-ionomer and resin-modified glass-ionomer) was exposed to aqueous solutions of differing pH for time intervals of a week, after which the pH of the storage solutions was determined. The results showed that all of the acid-base cements altered the pH of their storage solution, regardless of whether that initial solution was weakly acidic, weakly alkaline or close to neutral. All cements were found to act as buffers, because they not only increased the pH of the weakly acidic lactic acid solution, but they also decreased the pH of the weakly alkaline artificial saliva. In deionized water, the zinc polycarboxylate generally increased pH, while all other cements reduced it. In all cases, these results were shown to be repeatable on exposure to fresh-aqueous solutions of the appropriate pH for a further week, such experiments being carried out for up to six weeks. In terms of mass change, in most solutions, there was a modest increase during the first week, after which the mass remained steady. In lactic acid, zinc phosphate and zinc polycarboxylate cements showed a gradual reduction in mass throughout the six weeks, whereas the glass-ionomers showed an initial increase, followed by a much slower decrease in mass. These results confirm that glass-ionomers are the most resistant of the cements towards acid erosion.

Journal Article↗
PubMed1998

Changes on storage of polyacid-modified composite resins.

Two polyacid-modified composite resins, Dyract and Compoglass, have been studied for water-uptake on storage in three aqueous media, namely pure water, 0.9% NaCl and 1 M NaCl at 37 degrees C, and the results compared with those of a conventional composite resin, Pekafill. The equilibrium water-uptake of Dyract and Compoglass varied depending on time of cure and ionic strength of the storage medium. This latter finding contrasted with that for Pekafill, where equilibrium water content showed almost no variation with ionic strength of the storage medium. Uptake of water was always greater in Dyract and Compoglass than in Pekafill, reflecting the role of water in promoting the later neutralization reaction in these materials and its further role in hydrating the ionic products of that reaction. Despite this and other evidence of neutralization, specimens of Dyract and Compoglass stored in wet conditions (0.9% NaCl) showed no change in compressive strength on storage from 24 h to 30 days, a result similar to that reported previously for Dyract alone for flexural strength for times up to 1 year. By contrast, specimens of Dyract and Compoglass stored in dry air at 37 degrees C, and therefore unable to undergo any neutralization, showed an increase in compressive strength of between 40 and 70 MPa by 30 days that was significant at P < 0.01.

Air↗
PubMed1998

Glass-ionomers in medicine and dentistry.

This paper describes the current uses and future prospects for glass-ionomer cements in dentistry and medicine. Glass-ionomers divide into two chemical types, one is known as self-hardening and sets entirely by a neutralization reaction to give relatively brittle materials, the other is known as resin-modified and sets partly by polymerization and partly by neutralization to give slightly tougher materials. Compared with the self-hardening cements, these latter materials have improved aesthetics and easier clinical handling. Both types bond well to enamel and dentine, and release clinically useful amounts of fluoride. They have been used in a variety of applications in dentistry, including as liners/bases, luting cements for stainless steel crowns, and in various restorative procedures for both permanent and primary teeth. The resin-modified glass-ionomers are particularly promising for these latter uses, though it is too soon to be sure how durable they will prove to be over the longer term. Self-hardened glass-ionomers have been shown to have much better biocompatibility than resin-modified glass-ionomers in a variety of situations and consequently they have been used for various non-dental applications, such as ear, nose and throat surgery and craniofacial reconstruction.

Biocompatible Materials↗
PubMed1997

Changes in compressive strength on ageing in glass polyalkenoate (glass-ionomer) cements prepared from acrylic/maleic acid copolymers.

Previous studies have shown that glass-ionomers made from acrylic/maleic copolymers stored in water reach a maximum strength at about 1 week, and after 4 months have become significantly weaker. This finding, which contrasts with the behaviour of glass-ionomers based on poly(acrylic acid), was originally attributed to hydrolytic instability. This interpretation has been tested in the current work. Specimens of glass-ionomer prepared from acrylic/maleic acid copolymer have been stored for up to 4 months in different media, namely deionized water, dry air and vegetable oil, then tested for compressive strength. Specimens were in the form of cylinders of dimensions 6 mm high x 4 mm diameter, and storage temperature was 37 degrees C. Data were analysed using two-way analysis of variance (ANOVA) and in all three media specimens became weaker at 4 months than they had been at 1 week (P < 0.05). However, for the specimens stored in dry air and in water, the 1-week values were not the maximum. The fact that there was a loss of strength under all conditions led to the conclusion that it is not, after all, due to hydrolysis.

Acrylates↗
PubMed1997

The physics of water sorption by resin-modified glass-ionomer dental cements.

The water-sorption characteristics of two commercial resin-modified glass-ionomer dental cements (Baseline VLC, ex. Detrey Dentsply, and Vitremer lining cement, ex. 3M Dental Products) have been studied in more detail than previously. Water sorption in both cements proved to be rapid, reaching equilibrium at approximately 48 h for Baseline VLC and at approximately 10 d for Vitremer. Over the first 8 h or so, absorption was shown to follow Fick's law, with a diffusion coefficient of 1.56x10(-7) cm2 s(-1) for Baseline VLC (cured for 20 s) and 5.09x10(-7) cm2 s(-1) for Vitremer (also cured for 20 s). As expected, sorption of water was found to be faster in specimens cured for shorter cure times and slower for those cured for longer times. In the presence of sodium chloride, both at 0.9% and at 1 M, diffusion coefficients were significantly greater than in pure water, but did not vary significantly with sodium chloride concentration, being approximately 3.3x10(-7) cm2 s(-)1 for Baseline VLC and 8.0x10(-7) cm2 s(-1) for Vitremer. This is attributed to conformational changes in hydrophilic segments of the polymer on absorption of aqueous sodium chloride in which the molecules form more compact coils than in the presence of pure water. They thus create a microstructure that is more permeable to water. Sorption in salt solutions became non-Fickian much sooner than in pure water, i.e. at 3-4 h for both cements. This is probably due to concentration changes of salt within the cement, suggesting that these materials possess a degree of permselectivity. Finally, equilibrium water uptakes varied with salt concentration, being least in 1 M NaCl, which reflects the different chemical potentials of water in the various storage media.

Journal Article↗
PubMed1997

Glass-ionomer cements in restorative dentistry.

This article reviews the current status and future prospects for glass-ionomer materials. These materials are of two chemical types: the older, self-hardening cements, which set by an acid-base neutralization reaction to give relatively brittle materials; and the newer, resin-modified cements, which set partly by polymerization and partly by neutralization. Compared with the self-hardening cements, the latter materials have improved esthetics, improved resistance to moisture, and greater toughness. Both types of glass-ionomer cement bond well to enamel and dentin and release a clinically useful amount of fluoride. They have been used in a variety of applications: as liners or bases, for luting of stainless steel crowns, for Class V restorations in permanent teeth, and for Class II and Class III restorations in primary teeth. The resin-modified glass-ionomers are particularly promising for these latter uses, although it is too early to be sure whether their long-term durability is sufficient. Self-hardening glass-ionomer materials are likely to retain specific niches of clinical application, including in their metal-reinforced and cermet-containing forms.

Composite Resins↗
PubMed1996

The use of organic compounds of phosphorus in clinical dentistry.

Organic compounds of phosphorus have been developed for a range of applications in clinical dentistry. These include dentine bonding agents, restorative materials and therapeutic agents, such as active ingredients in anticaries mouthwashes. A characteristic feature of all of these applications is the good bonding of the phosphorus compound to the tooth. This review highlights the progress that has been made to date in preparing functional and durable organophosphorus bonding agents and cements, and includes coverage of recent synthetic work aimed at preparing improved organophosphorus molecules for this application. The review concludes that this remains a promising field of chemistry to explore in the search for improved, clinically useful dental materials.

Dental Bonding↗
PubMed1995

Effect of adhesive layer thickness on the bond strength of a zinc polycarboxylate dental cement.

This study reports the effect of varying the thickness of a commercial zinc polycarboxylate dental cement on its adhesion in a metal-to-metal single lap joint. The thickness was controlled externally in a specially designed apparatus, rather than by the inclusion of isodimensional glass spheres, as has been done previously. The results show that both the tensile shear bond strength and the mode of failure of the cement are influenced by the thickness of the cement layer in the bonded joints. Hence, these results confirm previous findings that there is an optimum thickness for the zinc polycarboxylate layer. They also show that failure is more likely to be cohesive with thicker layers. The optimum thickness of the adhesive layer was 205 microns for the particular cement studied, PolyF Plus (De Trey, Dentsply, Weybridge, Surrey, UK), for which the average shear bond strength was found to be 4.03 MPa. Using stress analysis, the maximum shear and tensile stresses, tau 12(max) and sigma 11(max), have also been obtained. These analyses showed that joints failed at the interface between the cement layers and the lap joint at adhesive layer thicknesses of below 100 microns. This resulted from the high stress concentrations in bonded joints with thin adhesive layers. On the other hand, thicker joints failed cohesively within the adhesive layer because of greater contributions from tensile stresses in these joints.

Adhesiveness↗
PubMed1993

Change in pH during setting of polyelectrolyte dental cements.

The change in pH during setting has been studied for five different glass polyalkenoate (ionomer) cements and for two different zinc polycarboxylate cements using a flat-headed combination electrode on both the fresh cement and on a slurry of the set cement. The results show that the pH of the glass ionomers was slightly lower in the early stages of setting than was the pH of the zinc polycarboxylates and also that the pH of glass ionomers rises more slowly. For anhydrous cements (i.e. those formulated from dried polymer) pH was found to rise quicker than for hydrous cements (i.e. those prepared from aqueous solutions of polymer). Previous workers have assumed that anhydrous cements undergo slower rises in pH than hydrous ones. Our results clearly refute this assumption, and also suggest that the reported pulpal irritation associated with the use of anhydrous glass ionomers may be due to something other than low pH.

Acrylic Resins↗
PubMed1993

New aspects of the setting of glass-ionomer cements.

For many years, glass-ionomer cements have been described as setting by the formation of a poly(acrylate) matrix. Recent research has suggested that a second reaction may be involved, namely, the formation of a silica matrix. So that this hypothesis could be tested, non-polymer cements, based on an ionomer glass plus acetic acid, were prepared and stored for up to six months. They were insoluble in water, and their compressive strength was found to increase rapidly over the period of storage. By contrast, the product of the reaction between ZnO and acetic acid was soluble in water. These results support the idea that there is a secondary setting reaction in glass ionomers and suggest that it is responsible for the increase in strength observed.

Acetates↗
PubMed1993

Influence of level of deoxynivalenol in the diet of dairy cows on feed intake, milk production, and its composition.

Eighteen primiparous Holstein cows were used in a 10-wk lactation study, preceded by a 2-wk covariate period, to determine the effect of concentration of deoxynivalenol in the diet on cow performance and transfer of deoxynivalenol and its metabolite, deepoxydeoxynivalenol, to milk. Diets were formulated to contain deoxynivalenol at 0, 6, and 12 mg/kg of concentrate DM, and daily intake of deoxynivalenol was .59, 42, and 104 mg, respectively. Increasing deoxynivalenol in the diet did not affect intake of concentrate or forage. Total milk output was not affected; however, milk fat responded quadratically; cows given deoxynivalenol at 6 mg/kg of concentrate DM had the lowest milk fat content and fat output. Overall energetic efficiency was not influenced because reduced energy output in milk was compensated by increased BW gains. No transfer of deoxynivalenol or deepoxydeoxynivalenol to milk was observed; concentrations were below detectable limits (1 microgram/ml) using HPLC-mass spectroscopy. We concluded that diets containing deoxynivalenol up to 6 mg/kg of dietary DM did not reduce feed intake of cows in this study and that deoxynivalenol or deepoxydeoxynivalenol was not transferred to milk. Further studies are required to confirm the apparent lack of effect of deoxynivalenol on milk production.

Animals↗
PubMed1992

The effect of using layered specimens for determination of the compressive strength of glass-ionomer cements.

Compressive strength is widely used as the criterion of strength of glass-ionomer dental cements, despite the difficulties in interpretation of the findings. With the introduction of light-cured glass-ionomer cements, which can be used only in thin layers, the question arises of how test specimens should be prepared for the measurement of compressive strength. A suggested method has been to prepare test pieces by building them up in layers, an approach which is examined critically in the current paper. Two different conventional (acid-base) glass-ionomers were studied with the use of layered and unlayered specimens of dimensions 6 mm (height) x 4 mm (diameter) and 12 mm (height) x 6 mm (diameter). While smaller samples gave the same value of compressive strength as larger specimens, layered specimens gave significantly lower values of compressive strength for both sizes. In view of these findings, and since the layered specimens are tedious to prepare, we conclude that compressive strength is unsatisfactory as a criterion of strength for light-cured glass-ionomer cements.

Acrylic Resins↗
PubMed1991

Glass-ionomer cements in orthopaedic surgery: design of laboratory tests.

Recently, interest has been shown in the use of Glass-ionomer cement as a bone cement in orthopaedic surgery. This paper discusses the determination of working time, setting time and compressive strength for an experimental Glass-ionomer [GI] and the existing poly(methyl methacrylate) [PMMA] cement. Conventional test methods and some novel techniques are explored. Suggestions are made for new techniques which will allow the comparison of different types of cement. The effect of different storage media on cement stability has shown that both cements show a degree of instability dependent on the medium used. The properties reported for the GI cement suggest that it has the potential to be a new bone cement.

Bone Cements↗
PubMed1991

The biocompatibility of glass-poly(alkenoate) (Glass-Ionomer) cements: a review.

The literature describing the biocompatibility of glass-poly(alkenoate) ('Glass-Ionomer') cements has been reviewed. This literature shows that these materials have generally good biocompatibility for both dental and orthopaedic use, this latter observation being very recent. There have, though, been a few reports showing that in certain circumstances these materials may cause pulpal irritation and the reasons for these particular findings are considered. Following discussion of the biocompatibility of Glass-Ionomer cements, consideration is given to the likely underlying causes of this feature. Three factors are identified as contributing to the biocompatibility of these cements. They are: (i) minimal exotherm on setting; (ii) rapid neutralization following mixing; and (iii) slow release of ions which are generally biologically beneficial or, at least, benign. This last point is considered in some detail. Previous studies of leaching of ions from Glass-Ionomer cements have shown that only inorganic species are released. The biological effects of each of these inorganic ions are described and their influence on biocompatibility discussed.

Aluminum↗