Mechanical properties of glass ionomer cements affected by curing methods
Introduction
Glass ionomer cements (GICs) are widely used in dentistry. The advantages compared to more conventional dental materials like composites, are good adhesion to tooth enamel and dentin,1., 2. good esthetics3 and long-term fluoride release.4., 5. However, these materials are limited in their applications due to low wear resistance, brittleness and low strength.6., 7., 8., 9., 10. Therefore their use is generally restricted to specific indications and restorations such as Class III and V lesions.11
GICs are formed by an acid–base reaction between a degradable alumino-silicate glass and an aqueous solution of polyalkenoic acid. The acid attacks and degrades the alumino-silicate glass structure, releasing calcium and aluminium cations. These cations are then chelated by the carboxylate groups and crosslink the polyalkenoic acid chains.12., 13. This crosslinking reaction is a continuous process evident by the increase in mechanical properties of the cement with time. The setting of GICs is in two stages. The initial stage, which produces the clinical set, occurs within the first 10 min after mixing. The second stage, involving the release of the calcium and aluminium cations within the matrix, is a slow and long-term continuation of the acid–base reaction. During the first reaction, the material is very sensitive to water uptake, while during the second step the material is very susceptible to dehydration. The short-term sensitivity to water, resulting in surface softening and as a consequence low wear resistance restricts the full potential of GICs for dental applications.
One way to overcome the dilemma of the short-term sensitive to water is the development of ‘command’ set materials. The resin modified GICs are similar to the conventional GICs, but benefit from the photo-polymerizable resin allowing the ‘command’ set by applying an external light source. These materials are much like the conventional GICs with respect to the good adhesion to tooth enamel and dentin.14 However, they have some of the disadvantages that are inherent of the use of resins. Polymerization shrinkage, swelling in aqueous media, toxicological problem related to monomer release, and poorer long-term mechanical properties compared to conventional GICs have been reported.15., 16., 17.
Fast or ‘command’ setting of conventional GICs can be accomplished by the addition of external energy such as ultrasonic excitation. Impressive clinical results with conventional GICs restoration by using an ultrasonic device were observed.18 A laboratory study, simulating a clinical size restoration, showed that ultrasonic excitation not only improves the instant set of the material, but also improves the hardness of the material within the first 24 h after setting.19 Because external application of ultrasonic excitation does not modify the chemical composition of the GICs it overcomes the disadvantage associated with the resin modified GICs.
In spite of a few reports19 on the effect of the application of the ultrasonic excitation on the setting time, and hardness of GICs, little is known about the curing mechanism, and strengthening of the material. Since a significant temperature rise in the GICs was observed with ultrasonic excitation, it was hypothesized that its effect on the setting of GICs may partially be explained by the heat effect.
The objective of this work was to assess the influence of externally applied command set applications on the mechanical properties of several commercially available conventional GICs.
Section snippets
Materials and methods
The restorative GIC used are listed in Table 1, together with the manufacturer and batch code data. The GICs were supplied in encapsulated form. The capsules were activated and mixed in accordance with the procedures supplied by each manufacturer. The capsules were mixed with a Silamat S5 (Vivadent, Schaan Liechtenstein). The compressive strength of the conventional GICs was measured under standard curing conditions (SC) and ultrasonic excitation (UC). Since a significant temperature rise in
Results
The results of the compressive strength measurements on Fuji IX FAST as a function of curing time and setting method are summarized in Table 2 and graphically depicted in Fig. 1. Also the result of the statistical analysis (ANOVA) between the groups SC, UC and HC are summarized in Table 2.
The results on the compressive strengths and the statistical analysis between the groups SC, UC and HC of, Fuji IX FAST, Fuji IX, Ketac Molar Quick and Ketac Molar are summarized in Table 3 and depicted in
Discussion
Anecdotal clinical results18 where the setting of the GICs was enhanced by the application of ultrasonic energy have been very impressive and do not agree with normal physical properties of these materials. In the first period of setting of the GICs the material is relatively weak in compressive and diametral tensile strengths and show high wear rates.6., 7., 8., 9., 10. On the other hand almost no difference in compressive and diametral tensile strengths are observed within a time span of 24 h
Conclusions
The objective of this work was to determine the mechanical properties of several commercially available conventional GICs, depending on external applied command set applications. The compressive strength of the conventional GICs was measured under standard curing conditions, ultrasonic excitation, and heat curing. From the measured compressive strength it can be concluded that a significant increase in mechanical properties for ultrasonic excitation and heat treatment compared to the standard
Acknowledgements
This research was supported by the European Community (ULTRASET # GRD1-2000-25152). Satellec is gratefully acknowledged for the use of the Suprasson P5.
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