Biomedical subjects

J W Nicholson

Publications and source records attributed to J W Nicholson.

At least 37 records · Page 2Linked to original sources

PubMed2003

The interaction of glass-ionomer cements containing vinylphosphonic acid with water and aqueous lactic acid.

A glass-ionomer cement containing an acrylic acid/vinyl phosphonic acid copolymer, has been investigated for its interaction with water and with aqueous lactic acid and the results compared with those from conventional glass-ionomers based on polyacrylic acid. Cylindrical specimens (12 mm high x 6 mm diameter) were placed in 8 cm3 of aqueous lactic acid (20 mmol dm(-3); pH 2.7) for 1 week, at the end of which the pH was determined. Each specimen was then placed in a fresh 8 cm3 volume of lactic acid and the pH determined after a further week. This procedure was continued for a total of 13 weeks for each specimen. Experiments were also carried out on similar specimens exposed to 8 cm3 of water for 1 week only. Further experiments were carried out in which discs of cement were exposed to thin films (0.15 mm) of lactic acid at pH 4.5, with pH values determined at 30 s, 1, 2, 5 and 10 min. After 1 week, cements had changed the pH of the lactic acid to a mean value of 3.63 (SD 0.08) while in weeks 2-13 they changed it to a mean value of 3.31 (SD 0.11). After 1 week in aqueous lactic acid the cements had gained mass by a mean of 1.26% (SD 0.59%) compared with 1.22% (SD 0.14%) in water. After 13 weeks in lactic acid, cements had lost a mean of 2.83% (SD 0.74%) in mass. Thin films of aqueous lactic acid changed from pH 4.5-5.1 at 30 s and to a steady value of 5.6 (SD 0.3) between 2 and 10 min. These results were similar to those for conventional glass-ionomer cements based on carboxylic acid polymers. Hence, it was concluded that the presence of the vinylphosphonic acid units made no significant difference to the interaction of cements with aqueous solutions.

Glass Ionomer Cements↗

PubMed2003

Changes in properties of polyacid-modified composite resins (compomers) following storage in acidic solutions.

The interaction of three polyacid-modified composite resins (compomers) with various acidic storage solutions, and also water, over periods of time up to 6 months has been studied and compared with those of a glass-ionomer and a composite resin. This interaction has been shown to vary in a complex way with length of storage and nature of the acid, and citric acid was found to be the most aggressive storage medium for glass-ionomer cement, and also for the compomers. The pure composite resin, by contrast, was relatively unaffected by all of the acid solutions examined. In all acids, the compomers showed a distinct buffering effect, i.e. they increased the pH towards neutral, as did the glass-ionomer. The extent of this also varied with duration of storage and nature of the acid. The biaxial flexure strength was determined and found to be essentially unaffected by the complex chemical interactions with acidic storage solutions. Values obtained for the compomers were lower than those of the composite resin, but above those of the glass-ionomer. Fourier-transform infrared (FT-IR) spectroscopy was employed to study the changes in the compomers following storage in the aqueous media, but bands were broad and no detailed assignments could be made. There were changes in the region of the spectra associated with metal carboxylates however, and this indicates that the secondary acid-base reaction had occurred following water uptake.

Acids↗

PubMed2001

Variations in the compressive strength of dental cements stored in ionic or acidic solutions.

The compressive strengths of various dental cements (a zinc polycarboxylate, a zinc phosphate, a glass-ionomer and two resin-modified glass ionomers, RMGICs) have been determined following storage in pure water, 0.9% sodium chloride solution or 20 mmol dm(-3) lactic acid solution for periods of time ranging from 24 h to 3 months. The glass-ionomer cement showed no differences between different storage solutions or at different storage times, whereas the zinc polycarboxylate, zinc phosphate and the resin-modified glass ionomer cements showed significant differences following storage in the solutions for 24 h compared with pure water. The zinc polycarboxylate cement was significantly weaker at 24 h in 0.9% NaCl and lactic acid than in pure water, whereas most of the other cements were significantly stronger in both 0.9% NaCl and lactic acid. One of the RMGICs (Vitremer luting, ex. 3M), however, was significantly stronger only in the NaCl solution, not in the lactic acid. In general, by 1 week, the strengths all reverted to being essentially the same as for specimens stored in pure water for most subsequent storage times, and did not change significantly on storage for up to 3 months. This effect of storage medium on the early strength has not been reported previously and since the media were chosen to model certain characteristics of natural saliva, the changes observed seem likely to occur in vivo. It is concluded that pure water is not the best medium for storing these cements if they are to behave as they do under clinical conditions.

Journal Article↗

PubMed2001

Effect of curing with a plasma light on the properties of polymerizable dental restorative materials.

Specimens of light-curable dental restoratives have been prepared using either a conventional dental curing lamp (for 20 or 30 s) or a plasma light (for 1 or 2 s). The specimens were then stored in water until their mass equilibrated, then dried to constant mass. Most specimens lost material in this process but the losses in all specimens cured with the plasma light were significantly greater than those cured with the conventional lights (P < 0.05). Longer cure times gave slightly reduced losses in water in most cases. The specimens were then returned to water and allowed to re-equilibrate and their equilibrium water uptake determined. There was no simple trend in this latter property because elution of loosely bound hydrophilic species may have resulted in a less hydrophilic specimen, whose equilibrium water content was therefore correspondingly lower. Overall, the losses through dissolution in water suggest that plasma curing is less effective for these materials than conventional light curing, as it probably results in material with lower molar mass. The losses for the resin-modified glass-ionomer were much greater than for other materials, and it was concluded that the more rapid polymerization with plasma light caused a significant inhibitation of the acid-base part of the setting process. These findings suggest that long-term durability of materials may be compromised by employing plasma light cure rather than a conventional cure system and further studies of this point are recommended.

Acids↗

PubMed2000

Measurement of shock wave rise times in metal thin films.

We have measured the rise time of laser-generated shock waves in vapor plated metal thin films using frequency-domain interferometry with subpicosecond time resolution. 10%- 90% rise times of <6.25 ps were found in targets ranging from 0.25 to 2.0 microm in thickness. Particle and average shock velocities were simultaneously determined. Shock velocities of approximately 5 nm/ps were inferred from the measured free surface velocity, corresponding to pressures of 30-50 kbar. Thus, the shock front extends only a few tens of lattice spacings.

Journal Article↗

PubMed2000

The rate of change of pH of lactic acid exposed to glass-ionomer dental cements.

The rate of change of pH of aqueous lactic acid at pH 4.2-4.5 (i.e. a little below that of active caries in vivo) in contact with disks of various commercial glass-ionomer cements has been determined in two configurations. In the first of them, a thin film set-up, 20 microl of solution was spread across the surface of a cement disk (diameter: 13 mm), and its pH determined by pressing a flat-ended electrode against the film at varying time intervals. In the second, a similar disk was immersed in 1.5 ml of solution, removed after varying time intervals, after which the pH of the solution was measured using a round-ended electrode. The latter measurement was more reliable, in that the pH electrode had time to equilibrate, whereas the former was more realistic because the film was approximately the same thickness as that of saliva on a tooth surface. Both series of experiments showed measurable differences in pH after only 30 s, with the thin-film configuration showing a range of pH changes of 0.5-1.2 units depending on the cement and the small volume configuration showing a range of 0.1-0.5 units, also depending on the cement. After 10 min, in the small volume experiments, the pH had generally increased further. The extent and speed of the change in pH led to the conclusion that ability of glass-ionomers to increase pH is likely to be an important mechanism of caries protection under clinical conditions.

Glass Ionomer Cements↗

PubMed2000

The effect of storage in aqueous solutions on glass-ionomer and zinc polycarboxylate dental cements.

Cylindrical specimens (dimensions 6 mm diameter x 12 mm height) of glass-ionomer and of zinc polycarboxylate dental cement have been stored in aqueous solutions for periods of 24 h, 1 week and 1 month. The solutions were of varying composition and affinity for water, and storage in them resulted in fluctuations in mass of the cements, an effect which was attributed to differences in the partitioning of water between the solutions and the cement specimens. Unlike the zinc polycarboxylate, the glass-ionomer gained mass in most of the solutions examined (except Na(2)SO(4)), showing it to have a much greater affinity for water than the zinc polycarboxylate. Despite the fluctuations in water uptake by the glass-ionomer, and loss of water by the zinc polycarboxylate, no statistically significant differences in compressive strength were recorded in any solution at any storage time. This contrasts with results reported previously for zinc polycarboxylates using smaller specimens, showing that specimen size has an influence on the interaction of cements with storage solutions.

Journal Article↗

PubMed2000

Factors affecting the ability of dental cements to alter the pH of lactic acid solutions.

Two related studies have been carried out to determine the effect of (a) powder:liquid ratio and (b) relative amounts of cement on the extent of buffering of a lactic acid storage solution using zinc polycarboxylate and glass-ionomer cements (both water-activated). The effect of varying the powder:liquid ratio was found to be slight and not statistically significant. On the other hand, increasing the numbers of specimens in a given volume of storage solution was found to have a significant influence on the final pH (at the 0.0005 level of significance). The fact that powder:liquid ratio did not affect the final pH led to the conclusion that attack occurs mainly at the matrix of these cements, confirming previous findings about the acid erosion process. The finding that increasing the relative amount of cement to acid storage solution strongly influenced pH suggested that, under clinical conditions, buffering would be important. When five specimens were used, the final pH corresponded to that of arrested caries, from which it was concluded that these cements have the ability to be cariostatic in vivo.

Acrylic Resins↗

PubMed1999

Storage of polyacid-modified resin composites ("compomers") in lactic acid solution.

OBJECTIVES: The aim of this study was to determine the interaction of four polyacid-modified resin composites with aqueous lactic acid solutions, and to compare changes with those for a glass-ionomer cement and a conventional resin composite. METHODS: For each material, namely Compoglass F, Dyract AP, Hytac and Ana Compomer, plus AquaCem (glass-ionomer cement) and Pekafil (conventional composite resin), five cylindrical specimens of 4 mm diameter x 6 mm height were prepared and weighed. They were stored individually in 2.0 cm3 of 0.02 mol l-1 lactic acid solution for 1 week then the pH was determined and the specimens reweighed. The lactic acid solution was replenished, and the specimens were stored for a further week, after which the pH and specimen weights were again measured. This was repeated at 1 week intervals until the specimens were 6 weeks old. Differences were analysed by ANOVA followed by Newman-Keuls post hoc analysis. RESULTS: All four polyacid-modified composites increased the pH of the solutions at all time intervals by at least 0.26 pH units (significant to at least p < 0.01). This effect was similar to that of the glass-ionomer (but significantly less, p < 0.05) while significantly greater (p < 0.05) than that for the composite, Pekafil, which, by contrast, had no effect on pH. The observed rise in pH reduced significantly over time (ANOVA, p < 0.05). After 1 week, all pH changes were accompanied by net reductions in specimen mass, indicating susceptibility to acid erosion. Hytac was significantly more resistant to this erosion than the other materials; conversely, it had the least effect on solution pH. SIGNIFICANCE: These results show that polyacid-modified resin composites neutralise lactic acid in vitro but suffer erosion in the process.

Analysis of Variance↗

PubMed1999

A preliminary study of the effect of glass-ionomer and related dental cements on the pH of lactic acid storage solutions.

Glass-ionomer cements, both self-hardening and resin-modified, have been shown to increase the pH of lactic acid solutions in which they are stored. Similar results have been obtained for a zinc phosphate and a zinc polycarboxylate cement. The pH was increased over a period of 7 days by between 1.54 and 2.65 pH units from an initial value of pH of 2.60, depending on the cement. It is concluded that, as a result of this ability to neutralize surrounding aqueous solutions, these materials may have the beneficial effect in vivo of inhibiting caries development. In the case of glass-ionomers, this mechanism might complement that of fluoride release.

Dental Caries↗

PubMed1999

Dental materials: 1997 literature review.

This review of the published literature on dental materials for the year 1997 has been compiled by the Dental Materials Panel of UK. It continues a series of annual reviews started in 1973. Emphasis has been placed upon publications, which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, ceramometallic restorations and resin-bonded bridges, ceramics, denture base resins and soft lining materials, impression materials, dental implant materials, orthodontic materials, biomechanics and image processing, resin composites, and casting investment materials and waxes). Three hundred and thirty three articles have been reviewed.

Dental Materials↗

PubMed1999

The effect of strontium oxide in glass-ionomer cements.

The reaction of strontium oxide powder with poly(acrylic acid) has been studied both alone and within glass-ionomer cements. Reaction was found to be slow and the strontium-carboxylate structure was found to be partially covalent in character, as determined by Fourier transform infrared spectroscopy (FTIR). These are similar to the structures formed by calcium in glass-ionomer cements, but are different from typical monomeric strontium carboxylates, which tend to be purely ionic. Strontium oxide powder introduced in two types of glass-ionomer cements, slowed down the setting reaction at both 21 degrees C and 37 degrees C, but at low levels (5 wt %), increased the compressive strength in both cement formulations studied. However, at higher levels, it was found to decrease the compressive strength. This study confirms the view that strontium is a cement-forming ion; but concludes that, except at very low levels, strontium oxide powder does not improve the properties of glass-ionomer cements.

Journal Article↗

PubMed1999

The long-term interaction of dental cements with lactic acid solutions.

A study of the interaction of dental cements with lactic acid solutions has been carried out in which individual cement specimens were repeatedly exposed to 20 mmol dm(-3) lactic acid for periods of a week. After each week of storage, the mass of the specimens was recorded and the pH of the solution determined. The glass-ionomers showed an initial increase in mass, followed by a decline that became steady from 6 weeks. Zinc polycarboxylate and zinc phosphate cements, by contrast, showed no early gain in mass, but eroded steadily more or less from the start of their exposure to lactic acid. For all cements, acid erosion followed linear kinetics, at rates ranging from 0.5%/week for the zinc phosphate to 0.28%/week for one of the glass-ionomers, Chelonfil (ESPE, Germany). At the end of six months, the zinc phosphate had lost 14.2% of its initial mass, the zinc polycarboxylate 9.9% and the glass-ionomers between 6.2 and 7.2%. Erosion was accompanied on every occasion by neutralization of the acid solution. Both erosion and neutralization continued steadily throughout the experiment. The effectiveness of neutralization was in the following order: zinc polycarboxylate>zinc phosphate>glass-ionomer. The pH change in Week 1 was much greater for the glass-ionomers and the zinc polycarboxylate than in all subsequent weeks.

Journal Article↗

PubMed1999

The properties of polymerizable luting cements.

The properties of a polyacid-modified composite resin and two resin-modified luting cements have been studied. The polyacid-modified composite resin had the slowest setting reaction and, in this respect, it did not conform to the current international standard for luting cements. The compressive strength of all of the materials was studied after varying periods of storage from 24 h to 1 year. The polyacid-modified composite resin showed a distinct dip in strength at 1 month in all of the storage media, but otherwise it showed no significant variation with either age or storage medium. The resin-modified glass-ionomers showed variation at 24 h with storage medium (deionized water, 0.9% NaCl or 20 mmol dm(-3) lactic acid), but thereafter they showed little variation, until 1 year, when Vitremer luting showed a significant decline in strength in pure water. However, at 24 h and when stored in water, all of the materials had strengths that easily exceeded the minimum requirement of the current standard (70 MPa). They all took up water on storage, with diffusion coefficients ranging from 1.32 to 17. 19x10(-7) cm2 s(-1). These values were found to depend on whether the specimens were stored in pure water or in physiological saline. However, equilibrium water contents varied only slightly between water and saline. The polyacid-modified composite resin, Dyract-Cem, took up the least water, as well as showing the smallest variation in strength with age. By contrast, it was more difficult to mix than the other materials and the high viscosity of the paste led to the formation of voids and other imperfections in the specimens.

Compomers↗

PubMed1998

Chemistry of glass-ionomer cements: a review.

Studies of the setting of glass-ionomer cements have been carried out for over twenty years, and there is now a considerable body of information concerning the steps that lead to the conversion of a freshly mixed cement paste into a solid, durable dental restorative. This paper reviews these studies, paying particular attention to more recent work. The conclusion is that glass-ionomers consist of interpenetrating networks of inorganic and organic components forming a matrix in which particles of unreacted glass are embedded. However, there remain uncertainties over aspects of the setting chemistry, for example over the role of (+)-tartaric acid in the setting reaction, and over the nature of the fluoride species which form during the reaction. The chemistry of resin-modified glass-ionomers is also discussed and shown to be more complex than that of the simple cements. The presence of the resin component slows down the ionic cure reaction of the conventional cement, and leads to both a significant exotherm and a set material capable of absorbing water reversibly. The paper concludes that the microstructure of the set cement depends completely on chemical composition and the kinetics of the setting process, and that an understanding of the setting chemistry of these materials is thus important for optimal clinical use.

Biocompatible Materials↗

PubMed1998

In vitro caries inhibition by polyacid-modified composite resins ('compomers').

OBJECTIVES: The aim of this study was to compare the in-vitro caries inhibition of two polyacid-modified composite resins (PMCRs). METHODS: Three standardized class V cavities were prepared in eight extracted human molar teeth. Two of these cavities in each tooth were restored with Compoglass and Dyract according to manufacturers' guidelines. A conventional glass-ionomer cement restoration (Chemfil II) was placed in the third cavity in each tooth as a control. The teeth were submerged in an acid gel (lactic acid, gelatin, thymol, pH 4.5) for 28 days and then resin embedded. Ground sections were examined at x 160 magnification for enamel surface lesion depth, dentine surface lesion depth, wall lesions and evidence of caries inhibition. RESULTS: There was no significant difference between the depths of enamel or dentine surface lesions between the two compomer and conventional glass-ionomer materials. However, both PMCRs exhibited greater wall lesions than the conventional glass-ionomer material indicating less caries inhibition. CONCLUSIONS: These results suggest that PMCRs provide less caries inhibition than glass-ionomer cements.

Anti-Infective Agents, Local↗

PubMed1998

Dental materials: 1996 literature review.

This critical review of the published literature on dental materials for the year 1996 has been compiled by the Dental Materials Panel of the UK. It continues the series of annual reviews started in 1973 and published in the Journal of Dentistry. Emphasis has been placed upon publications which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, resin-bonded bridges and ceramo-metallic restorations, ceramics, denture base resins and soft lining materials, impression materials, implants materials, orthodontic materials, biomechanics and image processing, resin composites and casting investment materials and waxes). Three hundred and thirteen articles have been reviewed.

Dental Alloys↗

PubMed1998

Dental materials: 1996 literature review. Part 2.

Ceramics↗