INTRODUCTION

Dental erosion is defined as a chemical process that involves the dissolution of enamel and dentin by acids not derived from oral bacteria, leading to a softening of those hard tissues.¹ First, erosive challenge induces enamel surface demineralization by allowing remineralization. Then, a long-term acidic attack causes irreversible loss of hard tissue, which is accompanied by a progressive softening of the surface.²

Currently, dental erosion is a frequent condition observed in clinical dentistry.³ Several methods can be used to prevent or slow the progression of dental erosion, such as dietary intervention, change in consumption of acidic beverages, oral hygiene, and the use of fluoride (F).⁴ Regarding F therapies, its effects are reported to be higher when applied at high concentrations, as has already been demonstrated with the use of dentifrices (5000 ppm F), gels (12,300 ppm F), and varnishes (22,500 ppm F).^5-7

However, the role of F-releasing restorative materials such as glass ionomer cements on the prevention of erosion is not entirely known. It is well established that these cements can release F for a prolonged time and additionally reincorporate it through topical applications, maintaining its preventive effect.⁸ Thus, considering that F released from glass ionomer cements can prevent the development of caries adjacent to restorations,⁹ it could also reduce the effects of erosion, preventing the defects in dental substrates adjacent to restoration margins.

However, there is a lack of studies showing the combination effect of different F-concentration dentifrices associated with restorative materials on the erosion process. Thereby, the possibility of finding a material or combination of materials that can reduce or prevent dental erosion becomes relevant. Therefore, this study aimed to evaluate the effect of high-F dentifrice in enamel demineralization adjacent to different restorative materials in vitro. The null hypotheses tested were that there would not be any effect of type of restorative material or dentifrice use on the response variable assessed.

METHODS AND MATERIALS

Restorative Procedures

Box-shaped standardized cavities (2 × 2 × 2 mm) were prepared at the center of each block with a cylindrical diamond bur (No. 1090, KG Sorensen, Barueri, SP, Brazil) replaced after every 8 preparations. The slots were made with high-speed rotation under water/airspray cooling. Enamel blocks were restored according to the manufacturer's instructions with the following materials: conventional glass ionomer cement (GIC), chemically activated resin-modified GIC, and nanohybrid resin composite as negative control (Table 1).

Table 1 Restorative Materials Used in the Study

^a 

Manufacturers' information.

GIC: glass ionomer cement, BIS-GMA: bisphenol A glycidyl methacrylate, UDMA: urethane dimethacrylate, TEGDMA: triethylene glycol dimethacrylate, BIS-EMA: bisphenol A ethoxylate dimethacrylate, HEMA: 2-hydroxyethyl methacrylate

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After placement of the material in the prepared cavity, the surface of the restorative material was covered with a polyester strip and a glass slab under pressure to expel excess material from the cavity. Ionomeric materials were hand mixed and placed in a single bulk with a syringe (Centrix Incorporation, Shelton, CT, USA). In addition, conventional GIC restorations were protected with varnish. For resin-modified GIC restorations, cavities were coated with Vitremer Primer (3M ESPE, St Paul, MN, USA). After primer polymerization, the GIC was placed and polymerized for 40 seconds. For composite resin restorations, cavities were etched for 15 seconds with 35% phosphoric acid (3M ESPE), rinsed thoroughly, and excess water was removed with disposable brushes (Microbrush, Grafton, WI, USA). The cavities were coated with the adhesive system (SingleBond, 3M ESPE). Composite was placed in three layers of 0.5 mm, each polymerized for 20 seconds with a photo-curing unit. Final restorations were additionally polymerized for 40 seconds.

For all restorations, finishing and polishing were carried out 24 hours later with aluminum oxide discs (SofLex System, 3M ESPE), with each disk applied for 15 seconds. For photo-activated materials, cavities were restored and light polymerized using a halogen-based light-curing unit (Optilux 400, Demetron Research Corporation, Danbury, CT, USA). The light output was tested (480±32 mW/cm²) before each use with a Demetron Model 100 radiometer (Demetron Research Corporation).

RESULTS

The erosive cycling model adopted in this study demonstrated the ability to demineralize the enamel surface, whereas significant differences (p<0.05) were observed between the initial hardness and the final hardness (after erosive challenge) for all study groups regardless of the material or dentifrice used (Table 2).

Table 2 Initial Vickers Hardness Number (IVHN), Final Vickers Hardness Number (FVHN), and Percentage of Surface Hardness Loss (%SHL) According to Dentifrices and Restorative Materials Used (Mean±SD, n=8)^a

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Table 2 Extended.

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The %SHL data (Table 2) showed significant effects for the isolated factors: material (p<0.0001) and dentifrice (p<0.0001), but not for the interaction (p=0.6416). Thus, lower %SHL was observed in the groups that received the treatment with 5000 ppm F dentifrice, regardless of the material used. Likewise, lower %SHL was observed when the tooth was restored with conventional GIC regardless of the dentifrice used.

DISCUSSION

Treatment for teeth with erosive wear can include minimally invasive therapies for multidisciplinary interventions. Current evidence shows that frequent applications of agents with high F concentration are considered potentially effective approaches to reduce erosive tooth wear.¹³ Moreover, in advanced cases of erosion, restorative treatment is necessary; in this case, selection of material depends on its esthetic properties, resistance to biodegradation, adhesive capacity, and F release.¹⁴ Thus, studies evaluating the effect of erosive substances on restorative materials are extremely relevant because most patients have at least one restored tooth and are subject to a contemporary diet containing many erosive substances.³

According to the results, mineral loss was found less often in groups treated with the 5000 ppm F dentifrice, regardless of restorative material used. These results agree with those of Moretto and others,⁵ who found that an experimental 5000 ppm F dentifrice was able to significantly reduce enamel erosion and erosion abrasion compared with a conventional 1100 ppm F dentifrice in vitro. Indeed, high-concentration F applications (such as oral rinses, gels, and varnishes) have been demonstrated to increase abrasion resistance and to decrease the development of erosion in enamel in other studies.^15,16 On the other hand, our findings disagree with those found by Rios and others,¹⁷ because in their in situ study, no significant differences were found among 1100 and 5000 ppm F dentifrices for enamel wear. However, it must be considered that a different protocol of treatment with dentifrices was used, because in their study, the exposure to F dentifrice slurry occurred for only 30 seconds, and subjects rinsed their mouths afterward with water.

In addition, regardless of dentifrice used, less mineral loss was found when the tooth was restored with conventional GIC. In fact, lower demineralization closer to GIC restorations was already reported in situ and could be explained by a high local F concentration.^18,19 Although the erosive loss of GIC was not evaluated in this study, it has been previously demonstrated that it may be followed by an increase in the pH of the acid solution. This is because of GIC's buffer capacity, which could also protect the tooth from mineral erosive loss.²⁰

Although the preventive action of F on dental caries is well documented,^21,22 its role in erosion is still a matter of debate. The present study, despite the limitations of an in vitro model, support a positive effect of combination F delivery methods on the enamel demineralization process by either caries or erosive challenge, thereby rejecting the null hypothesis raised.

CONCLUSION

In conclusion, use of a high-F dentifrice on teeth restored with conventional GIC provided an additional protection against enamel erosion; however, in situ and clinical trials are required to confirm the relevance of this combination.