Braz Dent J (1999) 10(1): 1-60 ISSN 0103-6440

| Introduction | Material and Methods | Results | Discussion | Conclusions | References |

This in vitro study evaluates the effect of storage time and acid etching on the tensile bond strength of glass ionomer cement to composite resins. The bonded assemblies were stored at 100% relative humidity and 37oC for 1 hour, 1 day, 1 week, 1 month and 3 months. The test specimen was loaded at tension to failure on an Otto Wolpert-Werke testing instrument with a crosshead speed of 6 mm/min. The results showed a significant statistical difference for etched Vidrion F when compared to etched Ketac Bond at all storage periods. The unetched samples were statistically similar at 3 months, with the highest values for Vidrion F.

Key Words: composite resin, glass ionomer cement, acid etching.

Introduction

    Composite resins have been widely used as restorative materials for anterior teeth, and more recently for posterior teeth, due to their enhanced physical and aesthetic properties. However, composite restorations present several technical problems. They do not ensure an adequate seal at the gingival margin, principally if that margin is on dentin (Fuss et al., 1990), because the composite shrinks when polimerized and does not bond sufficiently to the tooth without the use of bonding agents. McLean et al. (1985) proposed bonding composite restorative material to etched glass ionomer cement, the so-called sandwich technique, in order to reduce the bulk of the composite and take advantage of all desirable properties of the glass ionomer cement, such as fluoride release, biocompatibility, and micromechanical interlocking. The glass ionomer cement is used as the cavity liner, and the inner walls of the resultant cavity are acid etched and filled with composite resin. According to Quiroz and Lentz (1987), an unetched glass ionomer surface is unsuitable for bonding. Smith and Martin (1990) reported that an untreated cement surface would not offer micromechanical retention to composites.
     The principal manner of attachment of composite resins to glass ionomer cement is by mechanical interlocking (Norling and Duke, 1985; Mangum et al., 1990). Acid etching of the cement surface creates a mechanical interlocking because it causes considerable surface roughness by loss of matrix and the exposure of glass particles (Hassan and Nathanson, 1987; Smith, 1988; Wexler et al., 1988; Hinoura et al., 1991). Surface roughness is dependent on etching duration (Hassan and Nathanson, 1987) and cement maturity before acid etching (Smith and Martin, 1990). A continuing breakdown of the cement surface due to acid etching time was also noted by other investigators (Garcia-Godoy and Malone, 1986; Smith, 1988; Fuss et al., 1990; Smith and Martin, 1990; Mangum et al., 1990).
    In the early stages after initial set, the loss or uptake of water by the glass ionomer cement is common, resulting in shrinkage and loss of volume with a tendency to crack or gross distortion (Fuss, 1990). It can be speculated that a moderate degradation of the cement surface is desirable for suitable bonding to composites, consequently the success of the technique developed by McLean et al. (1985) which depends on etching time and cement maturity.
    The objective of this study was to compare the effect of storage and acid etching on the tensile bond strength of composite resin to glass ionomer cement.

Material and Methods

    The materials used were Herculite XR (Sybron/Kerr, Romulus, MI), a hybrid composite resin; XR Primer and XR Bond (Sybron/Kerr), a bonding system suitable for use with this type of composite resin; Vidrion F Glass Ionomer Cement (S.S. White, Rio de Janeiro, RJ, Brazil), an anhydride glass ionomer cement; Ketac Bond (ESPE, Seefeld, Germany), a conventional glass ionomer cement, and 37% phosphoric acid gel (Sybron/Kerr).
    The samples were divided into 4 groups (Vidrion F with and without etching and Ketac Bond with and without etching) of 5 repetitions each and stored for 1 hour, 1 day, 1 week, 1 month and 3 months at 37oC and 100% relative humidity. The ionomer cements were manipulated according to the manufacturers’ instructions, in the following proportions: Vidrion F: 0.107 g powder/0.046 ml liquid; Ketac Bond: 0.107 g powder/0.138 ml liquid. The mixture was applied to the interior of a cone-shaped cavity of a Teflon cylindrical matrix (2 mm in height and 5 mm outer diameter). A Teflon cylindric countermatrix was adapted to the matrix to provide 1 mm of overlap in height of the glass ionomer cement test specimens. The glass ionomer cement was cured for 20 min, and then phosphoric acid solution was applied to the cement surface for 30 s (Smith, 1988; Smith and Martin, 1990). Control samples were not subjected to this treatment. The specimens were then washed with tap water for 20 s and air dried for 20 s. Matrices were juxtaposed and fixed with a brass ring and the XR Primer was applied over the glass ionomer cement and cured for 10 s using a Visilux II activator light (3M Dental Products Division, St. Paul, MN). Over the cured XR Primer layer and XR Bond 1 mm of Herculite XR composite resin was applied. The composites and their respective bonding agents were cured for 40 s. Additional composite resin was applied to fill the mold, each of which was light-cured for 40 s. After 1 h, the mold assembly was removed from the brass ring and stored at 37oC and 100% relative humidity. After storage periods of 1 hour, 1 day, 1 week, 1 month and 3 months, the samples were tested in a universal testing machine (Otto Wolpert-Werke, Ludwigshafen, Germany) at a crosshead speed of 6 mm/min (Figure 1). The test specimen was loaded at tension to failure and the bond strength was calculated from the load required to cause debonding divided by the area of the adherent surface (19.62 mm²).

Results

    Tables 1 and 2 show the tensile bond strength for each etched and unetched product after different storage periods. These data showed that the highest tensile bond strength was obtained after 1 day, 1 week and 1 month storage for etched Ketac Bond with a statistical difference when compared to the 1 hour and 3 month storage periods, which presented the lowest tensile bond strength. The unetched Ketac Bond showed a stronger bond after 1 week storage.
    For the etched Vidrion F, the 3 month period presented statistically superior values when compared to 1 h storage period; however, acid etching did not improve the values of tensile bond strength of the composite resin to glass ionomer cement at the other storage times. Unetched Vidrion F presented statistical superiority only at the 1 week storage and the lowest results were obtained at 1 hour and 3 months.

Discussion

    Within the limits of this investigation, the data showed that the highest tensile bond strength was obtained at 1 week and 1 month, except for unetched Vidrion F at 1 day storage. A suitable explanation for this result is that after 1 and 24 hours of storage, maturation of the glass ionomer cement is incomplete, causing the lowest values of tensile bond strength. The effect of acid etching on immature cement is significantly higher, due to greater solubility of the superficial layer (Smith, 1988; Smith and Martin, 1990) reducing the strength of the bond (Wexler et al., 1988; Mangum et al., 1990). After 1 week storage, the improvement of the tensile bond strength was due to aging, sufficient time for complete cement maturation. The results for this storage period were the highest. However, the values for the etched specimens decreased during the storage periods of 1 and 3 months. This implies that hydrolysis of the bond may have occurred, decreasing the tensile bond strength. Another fact to be considered is the dimensional shrinkage of the cement due to aging, which may have modified the area of stress concentration, consequently weakening the bond in the composite-glass ionomer cement interface. However, for acid-etched Vidrion F glass ionomer cement, the bond strength values did not strongly decrease. According to Fuss et al. (1990), there is a considerable variation in the time required to achieve full maturation among cement products. This was confirmed in this study; the acid etching was not necessary to increase the tensile bond strength of both glass ionomer cements at some storage times. The surface acid treatment with variable degradation of the ionomer matrix was also influenced by the moist environment. Satisfactory adhesion is achieved when there is no hydrolysis of the interface bond.
    The best tensile bond strength was obtained without acid etching. Acid etching causes severe surface degradation for this type of cement resulting in poor tensile bond strength. Results obtained for Vidrion F were higher than those for Ketac Bond, which may be due to the cement conditions prior to acid etching, cohesive strength and particle size, all of which may affect bond strength. We suggest that acid etching not be used when glass ionomer cement is used as a lining base for composite resins.

Conclusions

    The present study indicated that both etched and unetched Vidrion F showed better tensile bond strength than Ketac Bond, and that acid etching did not improve the tensile bond strength of the two products.

References

Fuss J, Mount GJ, Makinson OF: The effect of etching on a number of glass ionomer cements. Austr Dent J 35: 338-344, 1990

Garcia-Godoy F, Malone WFP: The effect of acid etching on two glass ionomer lining cements. Quintess Int 17: 621-623, 1986

Hinoura K, Suzuki K, Onose H: Factors influencing bond strengths between unetched glass ionomers and resins. Operative Dent 16: 90-95, 1991

Hassan FS, Nathanson D: Shear bond strength of a composite resin to etched glass ionomer cement. J Dent Res 66: 132 (special issue), 1987

Mangum FI, Berry III E, Parikh UK, Ladd D: Optimal etching time of glass ionomer cement for maximum bond of composite resin. J Am Dent Ass 120: 535-538, 1990

McLean JW, Powis DR, Prosser HJ, Wilson AD: The use of glass ionomer cements in bonding composite resins to dentine. Brit Dent J 158: 410-414, 1985

Norling BK, Duke ES: Bond strength of a composite resin to a glass ionomer cement. J Dent Res 64: 315 (special issue), 1985

Quiroz L, Lentz DL: Laboratory evaluation of etching time on three different glass ionomers. J Dent Res 66: 131 (special issue), 1987

Smith EDK, Martin FE: Acid etching of a glass ionomer cement base: SEM study. Aust Dent J 35: 236-240, 1990

Smith GE: Surface deterioration of glass-ionomer cement during acid etching: an SEM evaluation. Operative Dent 13: 3-7, 1988

Wexler G, Beech DR, Chem C: Bonding of a composite restorative material to etched glass ionomer cement. Austr Dent J 33: 313-318, 1988

Correspondence: Correspondence: Dr. Marcelo Ferraz Mesquita, Avenida Limeira, 901, Bairro: Areião, 13414-018 Piracicaba, SP, Brasil. Telephone: +55-19-430-5200, Ext. 5296. E-mail: mesquita@fop.unicamp.br

Accepted November 11, 1998
Electronic publication: September, 1999