John W. Nicholson

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.

An evaluation of accelerated Portland cement as a restorative material

Biocompatibility of two variants of accelerated Portland cement (APC) were investigated in vitro by observing the cytomorphology of SaOS-2 osteosarcoma cells in the presence of test materials and the effect of these materials on the expression of markers of bone remodelling. Glass ionomer cement (GIC), mineral trioxide aggregate (MTA) and unmodified Portland cement (RC) were used for comparison. A direct contact assay was undertaken in four samples of each test material, collected at 12, 24, 48 and 72 h. Cell morphology was observed using scanning electron microscopy (SEM) and scored. Culture media were collected for cytokine quantification using enzyme-linked immunosorbent assay (ELISA). On SEM evaluation, healthy SaOS-2 cells were found adhering onto the surfaces of APC variant, RC and MTA. In contrast, rounded and dying cells were observed on GIC. Using ELISA, levels of interleukin (IL)-1beta, IL-6, IL-18 and OC were significantly higher in APC variants compared with controls and GIC (p<0.01), but these levels of cytokines were not statistically significant compared with MTA. The results of this study provide evidence that both APC variants are non-toxic and may have potential to promote bone healing. Further development of APC is indicated to produce a viable dental restorative material and possibly a material for orthopaedic

The biocompatibility of resin-modified glass-ionomer cements for dentistry

OBJECTIVES The biological effects of resin-modified glass-ionomer cements as used in clinical dentistry are described, and the literature reviewed on this topic. METHODS Information on resin-modified glass-ionomers and on 2-hydroxyethyl methacrylate (HEMA), the most damaging substance released by these materials, has been collected from over 50 published papers. These were mainly identified through Scopus. RESULTS HEMA is known to be released from these materials and has a variety of damaging biological properties, ranging from pulpal inflammation to allergic contact dermatitis. These are therefore potential hazards from resin-modified glass-ionomers. However, clinical results with these materials that have been reported to date are generally positive. CONCLUSIONS/SIGNIFICANCE Resin-modified glass-ionomers cannot be considered biocompatible to nearly the same extent as conventional glass-ionomers. Care needs to be taken with regard to their use in dentistry and, in particular, dental personnel may be at risk from adverse effects such as contact dermatitis and other immunological responses.

Studies on the structure of light-cured Glass-ionomer cements

The behaviour of two commercial light-cured glass-polyalkenoate (“Glass-ionomer”) cements has been studied in terms of changes in strength following storage under various conditions. Unlike conventional glass-polyalkenoates, these materials cure partly by a photochemical polymerization process, in addition to the normal acid-base setting reaction. Results presented in the current paper show that these hybrid materials are able to take up a considerable amount of water when stored in either water or physiological saline solution. This leads to an observed change of failure mode in compression from purely brittle to partly plastic. These changes are accompanied by a reduction in compressive strength. Overall, the behaviour of the light-cured Glass-ionomers was found to be similar to that of hydrogels. This finding is discussed in light of the information it gives about the underlying microstructure of these materials.

Glass-ionomer cements as adhesives

The literature on the clinical use of glass-ionomer cements is reviewed, and this shows that these materials are successful partly because of the good adhesion they exhibit towards a variety of substrates encountered in dentistry. The reasons for this good adhesion are identified as the good initial wetting of the surfaces met in clinical dentistry, the development of strong chemical bonding with the surface over time and the good mechanical properties of the cements themselves, which make them resistant to cohesive failure. In this review teese features are described in detail and related to established mechanisms of adhesion from the wider field of adhesives technology.

Studies on the setting chemistry of glass-ionomer cements

Abstract The Glass-ionomer cement is formed by the reaction of a polymeric acid and a basic glass. Conventional setting theory assumes that the process consists of the formation of a calcium/aluminium polyacrylate matrix, and makes no mention of the role of silica. Three different monomeric acids have been used to form weak cements, yet their calcium and aluminium salts are known to be water-soluble. This, together with the previous finding that silica is leached out of the glass during acid attack, suggests that the formation of a hydrated silicate is important in the setting reaction. The significance of such a silicate network within Glass-ionomer cements is discussed.

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.

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.

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.

Poisson's ratio of glass-polyalkenoate (“glass-ionomer”) cements determined by an ultrasonic pulse method

Poissons ratio has been determined for four glass-polyalkenoate cements; one experimental luting type, one experimental restorative type, one commercial restorative type (Chelon-Fil, ex. ESPE GmbH) and one cermet-reinforced cement [Chelon-Silver, also ex. ESPE]. An ultrasonic pulse method was employed for the Poissons ratio determinations. For the restorative materials, including the cermet-reinforced cement, the values obtained were all in the region of 0.3. For the luting cement, on the other hand, the values found were in the range of 0.35. This difference was consistent with the relatively increased water content and reduced glass content of the luting cement. The cements based on poly(acrylic acid) showed slight, but statistically significant, increases in Poissons ratio up to 80 days, whereas the cements based on poly(maleic acid) showed no significant changes in Poissons ratio with time.