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.

Spatially and spectrally resolved imaging of modal content in large-mode-area fibers

A new measurement technique, capable of quantifying the number and type of modes propagating in large-mode-area fibers is both proposed and demonstrated. The measurement is based on both spatially and spectrally resolving the image of the output of the fiber under test. The measurement provides high quality images of the modes that can be used to identify the mode order, while at the same time returning the power levels of the higher-order modes relative to the fundamental mode. Alternatively the data can be used to provide statistics on the level of beam pointing instability and mode shape changes due to random uncontrolled fluctuations of the phases between the coherent modes propagating in the fiber. An added advantage of the measurement is that is requires no prior detailed knowledge of the fiber properties in order to identify the modes and quantify their relative power levels. Because of the coherent nature of the measurement, it is far more sensitive to changes in beam properties due to the mode content in the beam than is the more traditional M(2) measurement for characterizing beam quality. We refer to the measurement as Spatially and Spectrally resolved imaging of mode content in fibers, or more simply as S(2) imaging.

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.

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.

The effect of Coca-Cola and fruit juices on the surface hardness of glass-ionomers and 'compomers'

The interaction of tooth-coloured dental restorative materials (a conventional glass-ionomer, two resin-modified glass-ionomers and two compomers) with acidic beverages has been studied with the aim of investigating how long-term contact affects solution pH and specimen surface hardness. For each material (ChemFil Superior, ChemFlex, Vitremer Core Build-Up/Restorative, Fuji II LC, Dyract AP and F2000) disc-shaped specimens were prepared and stored in sets of six in the following storage media: 0.9% NaCl (control), Coca-Cola, apple juice and orange juice. After time intervals of 1 day, 1 week, 1 month, 3 months, 4 months, 6 months and 1 year, solution pH and Vickers Hardness Number were determined for each individual specimen. Differences were analysed by anova followed by Student-Newman-Keuls post hoc analysis. All materials were found to reduce the pH of the 0.9% NaCl, but to increase the pH of the acidic beverages. The conventional glass-ionomers dissolved completely in apple juice and orange juice, but survived in Coca-Cola, albeit with a significantly reduced hardness after 1 year. The other materials survived in apple juice and orange juice, but showed greater reductions in surface hardness in these beverages than in Coca-Cola. Fruit juices were thus shown to pose a greater erosive threat to tooth coloured materials than Coca-Cola, a finding which is similar to those concerning dentine and enamel towards these drinks.

Fourier transform infrared spectroscopic study of the role of tartaric acid in glass-ionomer dental cements

(+)-Tartaric acid is incorporated into glass-ionomer dental cements to control the setting characteristics. FTIR has been used to examine the cements as they set, and has confirmed previous results that (+)-tartaric acid reacts more readily with the glass than does poly(acrylic acid), thereby delaying the setting of the cement. Subsequently, ions released by the glass become available for reaction with the polyacid, and gelation occurs. (+)-Tartaric acid not only reacts rapidly to yield calcium tartrate, but also enhances the rate at which aluminum polyacrylate is formed within the cement. Mesotartaric acid, by contrast, while rapidly forming calcium tartrate, does not enhance the rate of aluminum polyacrylate formation. As a result, this latter isomer merely suppresses the setting process; unlike (+)-tartaric acid, it does not also sharpen the set.

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.

A preliminary report on the effect of storage in water on the properties of commercial light-cured glass-ionomer cements

Two commercially available light-curable glass-ionomer cements, Vitrebond and XR-Ionomer, have been studied and their compressive strengths measured following storage under wet and dry conditions for varying lengths of time up to 3 months. The strongest cements were those stored in air and allowed to age. On the other hand, cements that were stored in water were found to become progressively weaker with time. Their failure mode was different from that of cements stored in air in that specimens became barrel-shaped as they were loaded and exhibited considerable plastic deformation prior to fracturing. By contrast, air-stored specimens behaved as predominantly brittle materials, the specimens essentially maintaining their integrity up to the point of catastrophic failure. Both of these findings indicate that the properties of these particular light-cured cements change markedly on exposure to moisture, a fact which is of clinical significance.

Buffering and ion-release by a glass-ionomer cement under near-neutral and acidic conditions

Specimens of an experimental glass-ionomer cement were stored in water (initial pH 5.9) and aqueous lactic acid (initial pH 2.7) for storage periods of 1 week up to 6 weeks. Change in mass, solution pH and fluoride release were measured at weekly intervals, and other ions were determined at weeks 1, 2, 4 and 6. In water, cements raised the pH consistently to 6.7-6.9 from weeks 2 to 6, but this did not correspond to uniform amounts of ions released, nor even to consistent mole ratios of ions in solution. Similarly, in lactic acid, pH was raised to between 3.6 and 4.5, but without a consistent concentration of ions in solution. In near neutral conditions, calcium was found to be virtually insoluble and remained within the cement, whereas reasonable amounts of sodium, aluminium, phosphorus, silicon and fluoride were released at all time intervals, with downward trends over time. In acidic conditions, considerable amounts of calcium were released over time, and amounts of calcium, aluminium, phosphorus and silicon increased with time, reaching a maximum in week 4. This suggests that as maturation proceeds, there is an increase in the acid-soluble fraction of the cement containing these elements. Fluoride release was found to be as previously reported, i.e. greater amounts in the early stages of the experiment, and with a gradual decline, and with greater amounts in acid than in water. Determination of fluoride with and without the decomplexing reagent TISAB showed that 70-75% of the total fluoride was released in "free" form in water for most weeks, whereas in acid, it declined sharply and by week 6, an estimated total of 96% of the fluoride released was complexed.

Enhancing the Mechanical Properties of Glass-Ionomer Dental Cements: A Review

This paper reviews the strategies that have been reported in the literature to attempt to reinforce glass-ionomer dental cements, both conventional and resin-modified. These cements are widely used in current clinical practice, but their use is limited to regions where loading is not high. Reinforcement might extend these applications, particularly to the posterior dentition. A variety of strategies have been identified, including the use of fibres, nanoparticles, and larger particle additives. One problem revealed by the literature survey is the limited extent to which researchers have used International Standard test methods. This makes comparison of results very difficult. However, it does seem possible to draw conclusions from this substantial body of work and these are (1) that powders with conventional particle sizes do not reinforce glass-ionomer cements, (2) certain fibres and certain nanoparticles give distinct improvements in strength, and (3) in the case of the nanoparticles these improvements are associated with differences in the morphology of the cement matrix, in particular, a reduction in the porosity. Despite these improvements, none of the developments has yet been translated into clinical use.

Prevention of enamel demineralization after tooth bleaching by bioactive glass incorporated into toothpaste

BACKGROUND

The aim of this study was to determine the effects of bleaching on the structure of the enamel layer of teeth and the potential of the commercial bioactive glass NovaMin® in two different toothpastes to remineralize such regions of the enamel. Three aspects were considered: the extent and nature of the alterations in the enamel after application of the bleaching agents; the extent of remineralization after application of two commercial toothpastes containing bioactive glass; and whether or not there were differences between the toothpastes in terms of their effectiveness in promoting remineralization.

METHODS

Bleaching agent based on 16% carbamide peroxide was applied to the enamel surface of freshly extracted human molars for 8 minutes, once a day for 7 days. After the bleaching cycles, the enamel surface was analysed by SEM and EDX.

RESULTS

The results obtained in the study lead to the conclusion that application of 16% carbamide peroxide causes distinct morphological changes to the enamel surface which vary from mild to severe. Subsequent treatment with either of the toothpastes containing the bioactive glass NovaMin® resulted in the formation of a protective layer on the enamel surface, consisting of bioactive glass deposits, with only slight differences between the two brands. Application of these dentifrices also caused increases in the Ca and P content of the enamel layer, returning it to that of undamaged enamel.

CONCLUSIONS

Remineralizing toothpastes should be used after bleaching, in order to repair any damage to the mineral tissue caused by these procedures.

The effect of diet composition and level of feeding on digestion in the stomach and intestines of sheep

1. Four wether sheep, each fitted with re-entrant duodenal cannulas and a rumen cannula, were used to determine the relative quantitative importance of digestion in the stomach compared with that in the intestines when diets of low (HM1) and high starch (CM1) content were fed at 0.9 times maintenance and when the high-starch diet was fed at 1.7 (CM2) and 2.3 (CM3) times maintenance. Paper, impregnated with chromic oxide, and polyethylene glycol (PEG) were administered.

2. An apparatus is described which allowed one operator to collect, record, sample and return the duodenal digesta from two sheep simultaneously.

3. Total digestion was determined by faeces collections over 7–10 day periods, and digestion in the stomach by measuring the total flow of digesta from the abomasum to the duodenum over two 24 h periods with two or three sheep on each ration. Flow values were adjusted to give 100% recovery of chromic oxide.

4. The recoveries of chromic oxide and PEG were similar and it was concluded that either marker was satisfactory for adjusting 24 h flow values. Disadvantages of using PEG are discussed.

5. There was relatively little difference between rations HM1 and CM1 in the proportion of digestible organic matter and energy digested in the stomach. Only 57.1% of the total dry-matter digestion occurred in the stomach for ration HM1 compared with 65.0% for ration CM1 reflecting a smaller net gain in ash for ration CM1. Doubling the level of intake (ration CM2) resulted in a decrease in the proportion of digestible dry matter, organic matter and energy digested in the stomach. When the level was increased still further with ration CM3, the proportion continued to decrease with one sheep but increased with the other.

6. All but 5–11% of the digestible starch (measured as α-linked glucose polymer) was fermented in the stomach, the lowest proportion being with ration HM1.

7. The amount of nitrogen reaching the duodenum was approximately equal to the amount ingested with rations HM1 and CM1 but considerably greater with the other two rations. All rations contained approximately 2% nitrogen.

8. Calculation of the estimated loss of energy as methane and heat of fermentation suggested that 50–54% of the digestible energy was absorbed as volatile fatty acids.

Polyacid-modified composite resins ("compomers") and their use in clinical dentistry

OBJECTIVES

This paper describes the chemistry and properties of polyacid-modified composite resins ("compomers") designed for use in clinical dentistry, and reviews the literature in this area.

METHODS

Information has been obtained from over 50 published articles appearing in the dental and biomaterials literature, with studies being principally identified through MedLine.

RESULTS

Published work shows that polyacid-modified composite resins constitute a discrete class of polymeric repair material for use in dentistry. Their distinction is that they contain hydrophilic components, and these cause water to be drawn into the material following cure. This triggers an acid-base reaction, and gives the materials certain clinically-desirable properties (fluoride release, buffering capability) that are also associated with glass-ionomer cements. The water uptake leads to a decline in certain, though not all, physical properties. However, clinical studies have shown these materials to perform acceptably in a variety of applications (Class I, Class II and Class V cavities, as fissure sealants and as orthodontic band cements), especially in children's teeth.

CONCLUSIONS/SIGNIFICANCE

Polyacid-modified composite resins constitute a versatile class of dental repair material, whose bioactivity confers clinical advantages, and which are particularly useful in children's dentistry.

Optically driven deposition of single-walled carbon-nanotube saturable absorbers on optical fiber end-faces

Optical radiation propagating in a fiber is used to deposit commercially available, single-walled carbon nanotubes on cleaved optical fiber end faces and fiber connectors. Thermophoresis caused by heating due to optical absorption is considered to be a likely candidate responsible for the deposition process. Single-walled carbon nanotubes have a fast saturable absorption over a broad wavelength range, and the demonstrated technique is an extremely simple and inexpensive method for making fiber-integrated, saturable absorbers for passive modelocking of fiber lasers. Pulse widths of 247 fs are demonstrated from an erbium-doped fiber laser operating at 1560 nm, and 137 fs pulses are demonstrated from an amplified Yb-doped fiber laser at 1070 nm.

Enamel alteration following tooth bleaching and remineralization

The purpose of this study was to compare the effects of professional tooth whitening agents containing highly concentrated hydrogen peroxide (with and without laser activation), on the enamel surface; and the potential of four different toothpastes to remineralize any alterations. The study was performed on 50 human molars, divided in two groups: treated with Opalescence(®) Boost and Mirawhite(®) Laser Bleaching. Furthermore, each group was divided into five subgroups, a control one and 4 subgroups remineralized with: Mirasensitive(®) hap+, Mirawhite(®) Gelleѐ, GC Tooth Mousse™ and Mirafluor(®) C. The samples were analysed by SEM/3D-SEM-micrographs, SEM/EDX-qualitative analysis and SEM/EDX-semiquantitative analysis. The microphotographs show that both types of bleaching cause alterations: emphasized perikymata, erosions, loss of interprizmatic substance; the laser treatment is more aggressive and loss of integrity of the enamel is determined by shearing off the enamel rods. In all samples undergoing remineralization deposits were observed, those of toothpastes based on calcium phosphate technologies seem to merge with each other and cover almost the entire surface of the enamel. Loss of integrity and minerals were detected only in the line-scans of the sample remineralized with GC Tooth Mousse™. The semiquantitative EDX analysis of individual elements in the surface layer of the enamel indicates that during tooth-bleaching with HP statistically significant loss of Na and Mg occurs, whereas the bleaching in combination with a laser leads to statistically significant loss of Ca and P. The results undoubtedly confirm that teeth whitening procedures lead to enamel alterations. In this context, it must be noted that laser bleaching is more aggressive for dental substances. However, these changes are reversible and can be repaired by application of remineralization toothpastes.

The physical properties of conventional and resin-modified glass-ionomer dental cements stored in saliva, proprietary acidic beverages, saline and water

Specimens of three conventional and one resin-modified glass-ionomer cement were prepared for both compressive strength and biaxial flexure strength determination. They were stored either in neutral media (water, saline, unstimulated whole saliva or stimulated parotid saliva) or in acidic beverages (apple juice, orange juice or Coca-Cola) for time periods ranging from 1 day to 1 year. In neutral media, the compressive and biaxial flexural strengths of all cements studied showed similar results, with significant increases apparent in compressive strengths at 6 months and which continued to 1 year, but no significant differences between the media; and no significant differences with time for biaxial flexure strength in all media. These findings show that interactions of these cements with saliva, which are known to result in deposition of calcium and phosphate, do not affect strength. Results for specimens stored in Coca-Cola were the same as for those stored in neutral media. By contrast, in orange and apple juice specimens underwent severe erosion resulting in dissolution of the conventional glass-ionomers after 3-6 months, and/or significant loss of strength at 1-3 months. Erosion of the resin-modified glass-ionomer, Vitremer, led to a significant reduction in strength, but not in dissolution, even after 12 months. The chelating carboxylic acids in these fruit juices were assumed to be responsible for these effects.

Maturation processes in glass-ionomer dental cements

Glass-ionomer cements are used for a variety of tooth-repair functions in clinical dentistry. They are formed by reaction of a basic glass powder with a solution of polymeric water-soluble acid, usually polyacrylic acid. After the initial neutralization reaction, by which the cement hardens, various maturation reactions occur. Changes induced by these maturation reactions are identified as: increase in strength; reduction in plasticity; improvement in opacity; and increase in proportion of tightly bound water. In addition, in contact with the tooth, an ion-exchange interfacial layer is gradually formed. This is mechanically strong and chemically-resistant. These changes are described in the current paper, which reviews the extent to which they occur, and reports what is know about the chemistry that underlies them. Processes involving slow diffusion of various ions and of water through the set cement bring about these changes. They include a secondary setting reaction to form a phosphate-based phase, binding of water to co-ordination sites around metal cations and to a hydration sheath around the polymer molecules, and possibly reaction of water with glass particle surfaces to form silanol groups. Evidence from a wide range of literature sources is used to be build up a detailed picture of the chemistry of the maturation processes, and gaps in our understanding are highlighted. The article concludes that, given the importance of glass-ionomers in contemporary dentistry, it is important to know the extent to which such maturation processes occur in current cement formulations, and also to determine how rapidly they take place.

Direct-comb molecular spectroscopy with accurate, resolved comb teeth over 43 THz

We demonstrate a dual-comb spectrometer using stabilized frequency combs spanning 177 to 220 THz (1360 to 1690 nm) in the near infrared. Comb-tooth-resolved measurements of amplitude and phase generate over 4×10(5) individually resolved spectral elements at 100 MHz point spacing and kilohertz-level resolution and accuracy. The signal-to-noise ratio is 100 to 3000 per comb tooth. Doppler-broadened phase and amplitude spectra of CO(2), CH(4), C(2)H(2), and H(2)O in a 30 m multipass cell agree with established spectral parameters, achieving high-resolution measurements with optical bandwidth generally associated with blackbody sources.

Review Paper: Role of Aluminum in Glass-ionomer Dental Cements and its Biological Effects

The role of aluminum in glass-ionomers and resin-modified glass-ionomers for dentistry is reviewed. Aluminum is included in the glass component of these materials in the form of Al2O3 to confer basicity on the glass and enable the glass to take part in the acid—base setting reactions. Results of studies of these reactions by FTIR and magic-angle spinning (MAS)-NMR spectroscopy are reported and the role of aluminum is discussed in detail. Aluminum has been shown to be present in the glasses in predominantly 4-coordination, as well as 5- and 6-coordination, and during setting a proportion of this is converted to 6-coordinate species within the matrix of the cement. Despite this, mature cements may contain detectable amounts of both 4- and 5-coordinate aluminum. Aluminum has been found to be leached from glass-ionomer cements, with greater amounts being released under acidic conditions. It may be associated with fluoride, with which it is known to complex strongly. Aluminum that enters the body via the gastro-intestinal tract is mainly excreted, and only about 1% ingested aluminum crosses the gut wall. Calculation shows that, if a glass-ionomer filling dissolved completely over 5 years, it would add only an extra 0.5% of the recommended maximum intake of aluminum to an adult patient. This leads to the conclusion that the release of aluminum from either type of glass-ionomer cement in the mouth poses a negligible health hazard.

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.

A study of the nature and formation of zinc polyacrylate cement using Fourier transform infrared spectroscopy

The formation and structure of zinc polyacrylate has been studied by Fourier transform infrared spectroscopy (FTIR). Using this technique we have shown that the fully hardened cement is bonded predominantly by zinc carboxylate complexes, rather than by associations of purely ionic character as had previously been supposed. Ionic bonds are formed in the initial stages of the reaction but are gradually converted to complex forms. This bond rearrangement appears partly responsible for physical changes that occur during gelation, setting, and hardening.

Ion release by resin-modified glass-ionomer cements into water and lactic acid solutions

OBJECTIVES

This study was undertaken to gain a fuller picture of the interaction of resin-modified glass-ionomers with aqueous solutions in terms of water sorption, solution buffering and ion-release.

METHODS

Two commercial materials were employed (Fuji II LC, GC; Photac Fil Quick, 3M ESPE). Light-cured cylindrical specimens (6mm highx4mm diameter) were prepared and stored for up to 6 weeks in either water (pH 5.3) or aqueous lactic acid (2.7), six specimens of each materials per storage medium. Solutions were changed at weekly intervals. Specimens were weighed at weeks 1, 2, 4 and 6; solution pH values measured, and Na, Ca, Sr, Al, P and Si ion release determined using ICP-OES. Results were analysed by ANOVA.

RESULTS

Materials altered solution pH, and gained mass under all conditions. However, the net mass gain in lactic acid declined with time, suggesting that water sorption was partly offset by erosion under acid conditions. Na, Ca, Sr, Al, P and Si were detected in all solutions, with greater amounts in lactic acid than in water. Phosphorus release was much lower than found previously for conventional glass-ionomers.

SIGNIFICANCE

Like their conventional counterparts, resin-modified glass-ionomers have been found to buffer their storage media and release ions. Hence the resin phase does not significantly alter these aspects of the interaction with aqueous solutions.