Microhardness of Enamel Restored with Fluoride and Non-Fluoride Releasing Dental Materials

Susana M.W. SAMUEL

Department of Dentistry, Faculty of Dentistry, Federal University of Rio Grande do Sul, Porto Alegre, RS, Brazil.

Correspondence: Profa. Dra. Susana M.W. Samuel, Departamento de Odontologia Conservadora, Faculdade de Odontologia, Universidade Federal do Rio Grande do Sul, Rua Ramiro Barcelos, 2492, 90035-003 Porto Alegre, RS, Brasil. e-mail: samuelsp@adufrgs.ufrgs.br

Braz Dent J (2001) 12(1): 35-38 ISSN 0103-6440


Demineralization caused by frequent ion loss can be detected by a decrease in enamel microhardness. The purpose of this study was to determine the microhardness of the enamel adjacent to restorations with fluoride and non-fluoride dental materials after demineralization and remineralization treatments using crowns of 6 recently extracted unerupted third molars which were sectioned into 4 parts. Five penetrations were made in the enamel of the control group and in the enamel adjacent to restorations made with Vitremer, Heliomolar or Z100, to obtain the Knoop microhardness. There was no significant statistical difference between the microhardness of the enamel of the control group and the enamel adjacent to the restorations made with Vitremer and Heliomolar (p<0.01), nor between the enamel adjacent to the Heliomolar and Z100 restorations. However, there was a statistically significant difference when the Z100 group was compared to the control group and to the Vitremer group. The results point out that some phenomena occurred which interferred with the ionic loss and, consequently, the microhardness of the enamel of the Vitremer and Heliomolar groups, because it was not possible to observe any difference between the microhardness of the enamel of the control group and the groups restored with Vitremer and Heliomolar.

Key Words: fluoride, demineralization, Knoop microhardness.


Dental caries is an infectious disease, which causes the local destruction of tooth hard tissues, and is associated to diet, microorganism accumulation and salivary conditions (1,2). The development of a clinically visible carious lesion is a consequence of the interaction of several factors of the oral environment and the dental tissues. Carbohydrate fermentation by bacteria of the dental plaque leads to the formation of various inorganic acids, causing a decrease in pH (1). When the pH of the oral environment reaches a critical value of 5.5, subsaturation of Ca+2 and PO4-3 ions occurs. The tendency is, thus, the loss of ions from the teeth to the environment, which is called demineralization. Frequent demineralization can lead to caries lesions (3). When the pH becomes higher than 5.5, through the buffering action of the saliva, there is a Ca+2 and PO4-3 supersaturation in the environment. In this situation, the tendency is for the teeth to incorporate ions, which is known as remineralization (4).

There is a constant ionic exchange between dental tissues and the environment, always seeking equilibrium. Studies have shown that the use of fluorides causes a decrease in caries (1,5). A number of investigations (1,2,4-7) have shown the importance of fluoride in demineralization and remineralization in controlling the development of caries, when fluoride is constantly present in the oral environment. Calcium fluoride is deposited on the dental surface when there is a decrease in the pH, working as a fluoride reservoir. Its solubility decreases when it is covered by a calcium phosphate layer, a compound derived from the saliva. When the pH decreases, the outer layer (calcium phosphate) dissolves, exposing the calcium fluoride, which is partially dissolved, releasing the fluoride (1). When fluoride is present, the critical pH for the occurrence of demineralization is altered from 5.5 to 4.5, that is to say, in an environment without fluoride there is a Ca+2 and PO4-3 ionic loss when the pH is 5.5, while if there is fluoride in the environment the ionic loss will occur only when the pH reaches 4.5 (4). Furthermore, fluoride leads to remineralization even before reaching the pH of 5.5 (1).

In clinical practice, time and money are wasted with the substitution of restorations, due to the recurrence of caries lesions. A means of reducing this significant problem would be the utilization of restorative materials which contain fluoride which could be released into the environment to promote remineralization (8). A number of in vitro studies have demonstrated a decrease in the incidence and severity of recurrent caries around restorations made with materials which release fluoride (7-10).

Featherstone et al. (11) and Kodaka (12) demonstrated the existence of a linear relationship between the square root of Knoop microhardness and the mineral content of dental tissues; therefore, the demineralization caused by frequent ionic losses could be detected by the reduction of the enamel microhardness. Thus, the purpose of this investigation was to determine the microhardness of the enamel adjacent to restorations made with fluoride and non-fluoride releasing dental materials after demineralization and remineralization.


Recently extracted unerupted third molars were stored in deionized water to maintain the condition of not having been in contact with fluoride. Six crowns were selected and were sectioned bucco-lingually and mesio-distally in the longitudinal axis, dividing them into 4 parts. One of these sections was stored in artificial saliva during the experiment (group 1: control). The other three sections received standard class V cavity preparation, in the center of the free surface, with a cylindrical diamond bur (number 1093, K.G. Sorensen, Barueri, SP, Brazil) and were restored with a) Vitremer (group 2), a resin-modified glass ionomer cement (3M do Brasil Ltda., Campinas, SP, Brazil), b) Heliomolar (group 3), a light-cured composite resin which contains ytterbium fluoride (Ivoclar, Vivadent do Brasil Ind. e Com. Ltda., Bonsucesso, RJ, Brazil), or c) Z100 (group 4), a light-cured composite resin without fluoride (3M do Brasil Ltda., Campinas, SP, Brazil). The Scotchbond Multipurpose adhesive system was utilized with the composite resins (3M do Brasil Ltda., Campinas, SP, Brazil). Each of the tooth fragments was painted with a coating of nail varnish and protected with wax to within 1 mm around the restoration. All fragments were stored in artificial saliva in individual containers.

The demineralization and remineralization cycles consisted of 4 daily cycles on 12 consecutive days. Initially, the fragments were maintained in individual containers for 30 min, immersed in a demineralizing solution of pH 4.3 according to Serra (7). After this period, they were washed with deionized water and immersed in artificial saliva (pH 6.8) for 3 h, and then replaced in the demineralizing solution for another 30 min.

After the conclusion of the demineralization and remineralization cycles, the wax and the nail varnish were removed and the fragments were embedded in acrylic resin with the aid of the Buehler specimen mounting press (Buehler Ltda., Evanston, IL, USA), so that after abrasion of the specimens it would be possible to visualize the tooth/restoration interface.

Finishing was done with 300- and 600-grit sandpaper, and water-cooled, followed by polishing with felt and pumice, also cooled with water, at room temperature.

The Knoop microhardness test was made with a NU Research Microscope (VEB Carl Zeiss Jena, Germany). Five penetrations were made with a load of 20 g in the enamel adjacent to the restoration, 50 mm from each other. The Knoop microhardness was obtained through the measure of the length of the major diagonal left by the penetration of the diamond, and calculated with the standard formula for Knoop microhardness (13).

ANOVA and Bonferroni tests were used for statistical analysis.


The results are presented in Table 1. Statistical analysis by ANOVA showed a significant difference between the microhardness of the enamel of the groups. The Bonferroni test showed that there was no significant statistical difference between the microhardness of the enamel of the control group and the Vitremer group. There was also no statistical difference between the microhardness of the enamel adjacent to the Vitremer and Heliomolar restorations (p<0.01), nor between the microhardness of the enamel adjacent to the Heliomolar and Z100 restorations. However, the Knoop microhardness of enamel adjacent to Z100 restorations was significantly different from that of the control group and the Vitremer group.


These results point out that some phenomena occurred which interfered with the microhardness of the enamel of groups 2 and 3, since it was not possible to observe any difference between the microhardness of the enamel of the control group and the groups restored with Vitremer and Heliomolar. A number of studies have investigated the action of fluoride in vivo and in vitro. Among them, Benelli et al. (14) reported that fluoride can delay caries development, even under high risk conditions. Erickson and Jensen (15) also verified that carious lesions were minor around restorations which released fluoride, and Swift (8) reported that fluoride released by restoration materials also affected the adjacent dental structure, decreasing enamel solubility.

Arends and Ruben (9) verified that the fluoride released by the Heliomolar composite was capable of enriching the surrounding enamel and/or dentin. There is a question about the long-term action of these materials, because the microhardness measurements were only made at an early stage. However, it is known that the glass ionomer cement can function as a fluoride reservoir (16-19) and can capture fluoride from the environment again, thus recovering its anticariogenic action with time. This situation probably does not occur with the composites, which after releasing the fluoride, do not restore it.


Based on the results of this study it is possible to conclude that, over the short term, materials with fluoride increased the microhardness of the enamel adjacent to restorations after demineralization and remineralization treatments.


The authors greatly appreciate the comments and help of Professor Robert G. Craig and Professor Léo Werner Süffert.


Samuel SMW, Rubinstein C. Microdureza do esmalte restaurado com materiais fluoretados e não-fluoretados. Braz Dent J 2001;12(1):35-38.

A desmineralização causada por freqüentes perdas iônicas pode ser detectada pela diminuição na microdureza do esmalte. A proposta deste estudo foi determinar a microdureza do esmalte adjacente a restaurações realizadas com materiais fluoretados e não-fluoretados, após processos de desmineralização e remineralização, utilizando 6 coroas de terceiros molares retidos, recentemente extraídos, que foram seccionadas em 4 partes. Cinco penetrações foram feitas no esmalte do grupo controle e no esmalte adjacente às restaurações feitas com Vitremer, Heliomolar ou Z100, para obtenção da microdureza Knoop. Não houve diferença estatística significativa entre a microdureza do esmalte do grupo controle e do esmalte adjacente às restaurações feitas com Vitremer e Heliomolar (p<0,01), nem entre o esmalte adjacente às restaurações realizadas com Heliomolar e Z100. Entretanto, houve diferença estatística significativa entre o grupo Z100 quando comparado ao grupo controle e ao grupo Vitremer. Os resultados indicaram que algum fenômeno ocorreu, o qual interferiu na perda iônica e, conseqüentemente, na microdureza do esmalte dos grupos Vitremer e Heliomolar, pois não foi possível observar nenhuma diferença entre a microdureza do esmalte do grupo controle e a dos grupos restaurados com Vitremer e Heliomolar.

Unitermos: flúor, desmineralização, microdureza Knoop.


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Accepted June 1, 1999
Braz Dent J 12(1) 2001