Fluoride release from glass ionomer cements and resin composites coated with a dentin adhesive
Introduction
Glass ionomer cements (GICs) are widely used in dentistry because of a variety of beneficial properties. Chemical diffusion-based adhesion to enamel and dentine, fluoride release, biocompatibility with tooth structure, simple application, esthetic appearance, acceptable abrasion resistance and capacity to be retained on unsupported enamel or non-undercut cavities are some of their favorable properties [1]. The two main reasons that have made GICs very popular are their permanent ionic bond to tooth structure, and their capacity to release fluoride, making them useful materials to replace dentin when used as bases in deep cavities [2], [3]. GICs have also been recommended for use as bases in conjunction with resin composites, and the so-called ‘sandwich’ or ‘laminate’ technique has become a common practical procedure to take advantage of GICs in combination with resin composites for restorations [4], [5].
When using a conventional GIC, its bond strength to the resin composite has been found to be limited by the low cohesive strength of the GIC [4]. Resin-modified glass ionomer cements (RM-GICs) have been recommended as bases to support an overlying resin restoration because of their enhanced physical and mechanical properties [2], [6]. In order to increase the bond strength of RM-GICs to dentin, and retain the other benefits when used in combination with resin composites in the laminate technique, it has been recently suggested that an adhesive bonding resin be applied to the dentin prior to the placement of a RM-GIC/resin composite restoration [3]. This study demonstrated that shear bond strengths of several RM-GICs were higher when a dentin adhesive was used, compared to their application directly to the dentin. However, the placement of a dentin adhesive prior to the application of a GIC results in the loss of direct contact of the GIC and the cavity wall. This ‘barrier’ may affect the ion exchange that normally occurs between a GIC and tooth structure when setting, which is responsible for the transfer of fluoride and other ions to the tooth.
It has been determined in many previous studies that GICs and RM-GICs have the capacity to slowly and steadily release fluoride over extended periods of time [7], [8], [9], [10], [11], [12], [13]. Amounts of fluoride release in deionized water have been found to range between almost 100 ppm to less than 10 ppm at 28 days in some conventional GICs [14], [15], between almost 50 to 7 ppm or less at 28 days in RM-GICs [7], [10], [14], and between 1 ppm or less to undetected limits (below 0.02 ppm) in some fluoridated resin composites [7], [15]. Fluoride release is an important property of those restorative materials, as it has been postulated to have anticariogenic potential by protecting both surrounding tooth structure and adjacent teeth against caries and demineralization [16], [17], [18], [19], [20]. It has also been postulated to enhance remineralization of early demineralized lesions of enamel [21], [22], [23], [24], [25], [26], and increase enamel resistance to subsequent acid attacks [23], [25], [26], [27]. Therefore, it would be desirable to continue placing them where associated with dentin margins of the cavity walls, compared with other materials. However, it is unknown whether the prior application of an adhesive resin to dentin affects the release of fluoride and its claimed anticariogenic potential. A previous laboratory study has reported fluoride release through fluoride-containing resin adhesives [28]. However, only one study has investigated fluoride release through surface coatings of non-fluoride-containing resin adhesives and varnishes [29].
The aim of this laboratory study was to compare the patterns and amounts of fluoride release from two GICs, two RM-GICs and two new fluoride-containing resin composites, coated with a layer of non-fluoride-containing dentin adhesive, to determine whether these surface coatings affect the capacity to release fluoride.
Section snippets
Materials and methods
The materials, the manufacturers and batch numbers used are shown in Table 1. A total of 72 cylinders, 4 mm in diameter ×6 mm long, were made using polyethylene molds. During setting, the bottom and top of the molds were covered by glass plates under hand pressure. Light-cured materials were polymerized for 20 s on both top and bottom surfaces. After polymerization, the cylinders were removed from the molds. For each material, six test specimens were coated with one layer of Scotchbond
Results
Patterns and amounts of cumulative fluoride release from uncoated and coated samples at days 1–28 are shown in Fig. 1, Fig. 2, and total release over 28 days in Table 2. The amounts of cumulative fluoride released from the uncoated samples, when comparing the materials at day 28, were statistically different between all materials, except for Fuji IX GP and Fuji II LC (p<0.05). The amounts of fluoride released at day 28 ranged from 2.3 to 85.4 ppm; the greatest was from Ariston pHc (85.4 ppm),
Discussion
Previous studies investigating fluoride release from GICs have used disc samples and expressed fluoride concentrations in units per area (mg/surface area). This has helped with comparisons among studies. However, volume ratios for fluoride release have been found to fit the same mathematical equation after varying the surface area of the specimen [30]. The goodness-for-fit of the curve fitting for fluoride release is the same for samples of a thickness of 3, 5 or 10 mm [30]. These two aspects
Acknowledgements
The authors wish to acknowledge the considerable assistance and advice of Dr Stuart Dashper, Oral Health Sciences Unit, School of Dental Science, The University of Melbourne, regarding the fluoride analyses.
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