An in vitro investigation of a poly(vinyl phosphonic acid) based cement with four conventional glass-ionomer cements. Part 1: Flexural strength and fluoride release.

OBJECTIVE: To investigate the flexural strength and fluoride release of four conventional glass-ionomer cements: Ketac-Molar (KM), HiFi (HF), Vivaglass Fil (VF), Ketac-Fil (KF) and a newly developed glass polyphosphonate cement, Diamond Carve (DC). METHOD: Disc specimens (10 mm diameter, 1 mm thick) were prepared and mould stored at 37 degrees C. After one hour, the specimens were removed from their mould and immersed in 20 ml of deionised water until required for testing. Biaxial flexural strength was determined at 1 hour and at 1, 7, 30 and 90 days after the start of mixing. Measurements of fluoride release from the specimens were carried out at 2 hours and at 1, 3, 7, 14, 30, 60 and 90 days after the start of mixing using a fluoride ion selective electrode. The results were analysed using ANOVA and student 't' tests. RESULTS: All the materials displayed different flexural strength patterns. KM and DC became stronger whilst KF and VF plateaued in strength with time. HF peaked in strength and then became weaker. At 90 days, the mean flexural strengths in decreasing order was as follows: KM > or = VF > or = DC > or = HF > KF. An initial fast rate of fluoride release followed by a slower but steady release of fluoride was observed in each of the materials. The mean cumulative fluoride release in decreasing order was as follows: VF > KF > or = HF > DC > KM. VF released significantly higher level and KM significantly lower level of fluoride than the other materials. CONCLUSIONS: The acid used to form the cement could not be used to predict changes in cement strength behaviour with respect to time. DC increased in strength with time and its flexural strength at 90 days was comparable to that of HF and VF. The cumulative and rate of fluoride release varied for the materials. DC had a low fluoride release consistent with a fast setting material with good early resistance to water.

An in vitro investigation of a poly (vinyl phosphonic acid) based cement with four conventional glass-ionomer cements Part 2: Maturation in relation to surface hardness.

OBJECTIVE: To investigate the surface hardness of four conventional glass-ionomer cements: Ketac-Molar (KM), HiFi (HF), Vivaglass Fil (VF), Ketac-Fil (KF) and a newly developed glass polyphosphonate based cement, Diamond Carve (DC) at different maturation times in water and to investigate the effects of early water exposure on their surface hardness. METHOD: Disc specimens (10 mm diameter, 1 mm thick) were prepared and mould stored at 37 degrees C. The effect of different maturation times (15, 30 and 60 min) and storage in water over 24 h after those maturation times on surface hardness was determined using a microindentor with a Vickers diamond indentor. The results of the surface hardness tests were analysed using Mann-Whitney non-parameteric statistics (p < or = 0.05). The working (WT) and setting (ST) times of the cements were also measured using a modified Wilson oscillating rheometer. RESULTS: All the materials became harder after 24 h immersion in water. HF, VF and DC showed initial sensitivity to a short maturation time, but only HF was adversely affected by early moisture exposure. KF and KM were least sensitive to short maturation time or early water exposure. DC had the shortest and HF the longest WT and ST. HF and VF had a high WT:ST ratio of 1:7 and 1:9, respectively. CONCLUSIONS: A short maturation time (of 15 min) and early exposure to water did not adversely affect the surface hardness of KF, KM, VF and DC. DC, based on poly(vinyl phosphonic acid), had the shortest WT and ST. Poly(acrylic acid) based HF and VF had a long ST in relation to their WT.

The effect of curing regime on the release of hydroxyethyl methacrylate (HEMA) from resin-modified glass-ionomer cements.

This study investigated the effects of degree of cure and specimen maturation on the release of HEMA from four resin-modified glass-ionomer cements. Disc specimens were light-cured for either the manufacturers' recommended time (MRT) for that material, or were under-cured (irradiated for 0.5 MRT), over-cured (1.5 MRT) or allowed to dark-cure (0 MRT). Specimens were matured in the mould at 37 degrees C until set or for 10 min, 40 min or 24 h prior to immersion in distilled water (at 37 degrees C) for 4 h. The HEMA release was determined by measuring the HEMA concentration in the storage water by HPLC. Vitremer, Fuji II LC and Fuji Lining LC set without light-curing (0 MRT) in less than 6 min and these specimens released low levels of HEMA (approximately 1, 7 and 4% of the included HEMA, respectively). Dark-cured Vitrebond took 15 min to set and the HEMA release was high (approximately 50% of the included HEMA) indicating that the material had only set by the acid-base reaction. HEMA release from light-cured Vitremer or Vitrebond was not affected by under- or over-curing but release from Fuji II LC was reduced by over-curing (1.5 MRT). Under-curing Fuji Lining LC caused a significant increase in HEMA release. Specimen maturation times of 10 min, or longer, had little effect on the measured HEMA release. All the materials released HEMA into their storage solutions. In order to minimise HEMA release, resin-modified glass-ionomers should always be cured for at least the manufacturers' recommended time at thicknesses no greater than the maximum recommended.

Long-term flexural strengths of resin-modified glass-ionomer cements.

The use of resin-modified glass-ionomer cements (RMGICs) in restorative dentistry has expanded since their introduction about seven years ago. These materials have been shown to possess superior initial mechanical properties to those of the conventional glass-ionomer cements. However, studies on the strength of RMGICs as a function of storage time are limited. This in vitro study investigated the effects of long-term storage of RMGICs in two storage media, distilled water and artificial saliva, at 37 degrees C on their four-point and bi-axial flexural stengths for a period up to 1 yr. All materials, except Fuji Lining LC, were weak at 5 min after light exposure but reached the maximum strengths within 7 d. Subsequently, there was a small decline in strength. Generally, the specimens aged in distilled water and artificial saliva had similar strength values. The bi-axial strengths of RMGICs were significantly greater than those obtained from the four-point flexure tests by a factor of approximately three. More variations in strengths of RMGICs with time were also observed from the four-point flexure test than from the bi-axial flexure test.

Long-term surface micro-hardness of resin-modified glass ionomers.

OBJECTIVES: The presence of a resin component in resin-modified glass-ionomer cements (RMGICs) results in considerable water sorption which may affect the surface hardness of the materials. This study investigated the effect of long-term storage in aqueous solutions on the surface hardness of RMGICs. METHODS: The surface micro-hardness of four RMGICs stored in either distilled water or artificial saliva was measured using the Wallace microindentation tester at regular time intervals up to 360 days. RESULTS: There was a substantial increase in hardness during the first day of storage when RMGICs were kept in distilled water. All RMGICs except one liner/base material reached their maximum hardness within 7 days and maintained their hardness for up to 1 year. RMGICs stored in artificial saliva were relatively softer and showed a decrease in surface hardness with time. CONCLUSIONS: The post-hardening reaction overcame the plasticising effect of water when RMGICs were stored in distilled water. However, progressive and high water uptake of specimens stored in artificial saliva resulted in a decrease in their surface hardness.

The uptake and release of fluoride by ion-leaching cements after exposure to toothpaste.

OBJECTIVES: The cariostatic action associated with the glass-ionomer cement (GIC) is usually attributed to its sustained release of fluoride. However the ability of the GIC to act as a fluoride reservoir, taking it up from an external source (e.g. toothpaste, mouthwash) and subsequently releasing it over time, may also be a contributory factor. This study investigated the reservoir effect of various recently introduced ion-releasing cements: two resin-modified glass-ionomer cements (Fuji II LC, Vitremer), a compomer (acid-modified composite resin) (Dyract), and a recently introduced conventional glass-ionomer (Fuji IX). METHODS: Specimens were exposed to a fluoridated toothpaste after 28 and/or 58 days. The release of fluoride into the storage water, both before and after exposure, was monitored using a differential electrode cell. RESULTS: There was no significant difference in the fluoride releases from Vitremer and Fuji II LC. These materials released significantly more fluoride than Fuji IX and Dyract. All the materials released more fluoride on the day after exposure to an external fluoride source compared with the day before exposure. Release rates returned to baseline within 3 days. Within the time periods of the study, only the uptake/re-release of Fuji IX was adversely affected by late exposure. All the materials showed an enhanced uptake and release on repeated exposure to the external fluoride source. CONCLUSIONS: All the materials under test (Dyract, Fuji II LC, Vitremer and Fuji IX) released significant amounts of fluoride and reacted positively to exposure to an external fluoride source.

Long-term flexural strength of three direct aesthetic restorative materials.

OBJECTIVES: This investigation evaluates the long-term flexural strength behaviour of three different glass-ionomer cement (GIC) based materials. METHODS: The materials under investigation were a conventional GIC (Opusfil), a metal-reinforced GIC (Opus Silver) and a resin-modified GIC (Fuji II LC). Flexural strength specimens of the materials were prepared according to the relevant manufacturers' instructions for clinical use. After 10-min maturation at 37 degrees C, the specimens were placed in water, also at 37 degrees C, until required for testing. Specimens were tested under four-point bend at intervals between 7 and 130 days after manufacture. RESULTS: Opusfil increased in strength up to day 56, but was significantly weaker after this time. Opus Silver was strongest on day 7, although there was no further decline in strength after day 14. Fuji II LC increased in strength up to day 14, but there was deterioration in strength after day 100. Opus Silver had the highest early strength of the three materials, but after day 7 there was no significant difference in strength between the three materials under test. CONCLUSIONS: The three cements tested did not exhibit the continued increase in strength with time that has been attributed to glass-ionomer cements. Each material behaved differently, but they all appeared to be adversely affected by storage in aqueous media. Materials formed by the inclusion of metal or the incorporation of resins into the glass-ionomer cement formulation should be regarded as separate sub-classes of materials in their own rights.

Water sorption characteristics of resin-modified glass-ionomer cements.

When restorative materials take up water, their dimensions and structural integrity may be affected. This study determined, using gravimetric measurements, the water sorption characteristics of four resin-modified glass-ionomer cements (RMGICs) immersed in either distilled water or artificial saliva. The dimensional changes on water storage were also determined. The RMGICs exhibited differing characteristics as they absorbed water. Percentage water uptake and solubility of the liner/base RMGICs were significantly greater than those of the restoratives. Higher rates of water uptake, i.e. higher values of sorption diffusion coefficients, were found in the materials from one manufacturer. The desorption diffusion coefficients of the four cements were not significantly different from one another, but were significantly higher than the sorption coefficients, indicating rapid water loss from the desiccated cements. All the cements expanded on immersion in water and contracted during desorption. Greater weight increase and greater volumetric expansion were observed on long-term storage in artificial saliva than in distilled water.

Factors contributing to the temperature rise during polymerization of resin-modified glass-ionomer cements.

Part of the setting reaction of a resin-modified glass-ionomer cement (RMGIC) is a photoinitiated polymerization. As a result of the polymerization exotherm, the temperature of the cement may rise during setting. This study investigated the temperature rise for two liner/base- and two restorative-type RMGICs. The effects of factors such as specimen thickness, exposure time and environment temperature were investigated. The thermal diffusivity of the cements was also evaluated. Temperatures were measured using a thermocouple embedded in the centre of 6-mm diameter specimen discs of 1, 2, or 3 mm thickness. The exposure times used to cure the specimens varied from 15 to 60 s. The test were carried out at either 25 or 37 degrees C. The temperature rises attributable to the polymerization reaction ranged from 11 to 26 degrees C for the liner/base cements and from 8 to 17 degrees C for the restorative cements. Increasing the specimen thickness reduced the temperature rise only when inadequate exposure times were used. Raising the environmental temperature resulted in a smaller temperature rise. The thermal diffusivities were determined from cylindrical specimens. These ranged from 1.9 x 10(-3) to 2.5 x 10(-3) cm2 s-1, the lining cements showing lower values than the restorative materials.

Method for enhancing the fluoride release of a glass-ionomer cement.

The cariostatic action of the glass-ionomer cement has been attributed to its sustained release of fluoride. The fluoride in the set cement originates from the glass particles which are eroded, in part, during the setting reaction. In this study a water-activated glass-ionomer was mixed with sodium fluoride (NaF) solutions of different concentrations (0%, 2% and 4%). The different cements were used to prepare discs which were stored individually in demineralized water at 37 degrees C, the fluoride concentration of the storage solutions were measured, at intervals, until the specimens were 80 days old. The effect of the different mixing solutions on the working and setting times and compressive strength were also determined. It was found that the cement mixed with the 4% solution of NaF released significantly more fluoride than the water mixed control. The fluoride solution mixed materials had longer setting times than the control, but there was no significant difference in the compressive strengths. All the materials became progressively stronger on storage. Mixing the cement with a 4% solution of NaF increased the initial fluoride release of the glass-ionomer without seriously affecting other physical properties.

Temperature rise in ion-leachable cements during setting reaction.

Resin-modified ion-leachable cements have been developed for use as aesthetic restorative materials. Their apparent improved physical and handling properties can make them more attractive for use than conventional glass-ionomers. However, they contain monomers which are known to contract on polymerization and produce a polymerization exotherm. This study evaluated the temperature rise during setting and the rate of dimensional change of several ion-leachable materials. The resin-modified ion-leachable cements demonstrated greater temperature rises and higher rates of contraction than conventional materials. Generally, the behaviour of these resin-modified materials was similar to that of composite resins. However, some resin-modified cements produced a temperature rise of up to 20 degrees C during polymerization which was greater than that of the composite resin. This temperature rise must be taken into account when using the materials in direct contact with dentine in deep cavities without pulp protection. Longer irradiation time than the recommended 20 s did not significantly increase the maximum temperature rise but slightly extended the time before the temperature started to decline. The temperature of the environment had a significant effect on the rate of dimensional change in some materials. The rate of polymerization contraction of light-activated cements was directly related to the observed temperature rise.

Influence of humidity on dimensional stability of a range of ion-leachable cements.

The dimensional changes of a variety of dental restorative materials, occurring during and after setting, were investigated. The materials were tested under four different environmental conditions: 25 degrees C at laboratory humidity, 25 degrees C at 100% humidity, 37 degrees C at laboratory humidity and 37 degrees C at 100% humidity. Two materials setting by an acid-base reaction were also examined when protected with both a conventional varnish and a low-viscosity light-curable resin. The dimensional changes were recorded continuously using linear variable displacement transducers (LVDTs) over periods of up to 2 h. The materials investigated showed varying magnitudes of dimensional change. The shrinkage of conventional glass-ionomer cements (Fuji II and Opusfil) were the highest at 37 degrees C in air. This was attributed to the highest rate of water loss in the most desiccating environment. The shrinkage observed for the materials which set, even only in part, by a polymerization reaction will probably be due to the water loss and/or polymerization shrinkage. Exposure of these materials to a high-humidity environment reduced the shrinkage because of the swelling associated with water absorption. Application of the varnish and the protective resin over the cement surfaces also reduced shrinkage in Fuji II due to prevention of water exchange. The apparatus used in this study provided a simple and reliable method for measuring linear dimensional change. Data obtained in this study were comparable, where appropriate, to the values found in the literature.

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.

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.