Introduction

The development of dental caries has seen a growing reduction due to the use of fluoride (F) not only in public water supplies but also in other fluoride vehicles, such as toothpastes [1, 2]. The F acts as an important adjunct to the maintenance of the mineral balance of teeth, acting directly on the dynamics of the de-remineralization process, inhibiting demineralization, promoting remineralization, and forming calcium fluoride [3]. Despite the existence of numerous sources of F, there is currently a trend in polarization of caries, with 25 % of the subjects having dental caries in approximately 75 % of their teeth [4]. Studies have shown that this bias is particularly associated with socioeconomic factors [5, 6]. This indicates, therefore, that individuals such as children [7], teenagers, and people who do not have access to fluoridated water and dental services [8] are at a high risk of developing caries.

It is known that fluoridated toothpastes (1100 ppm F) are among the most widely used forms of topical administration in the world and contribute majorly to the reduction of tooth decay [3]; it would be beneficial to increase their efficacy, which would decrease the levels of disease. A method used to increase the effectiveness of toothpaste is the addition of phosphate salts. Studies have shown that the addition of polyphosphates to fluoride formulations with low fluoride concentrations increased their effectiveness against demineralization [9–17]. This effect is associated with the ability of phosphates to form complexes with cations such as Ca2+ and CaF+ and increase the ionic activity of neutral species such as CaHPO4 0 and HF0 [10, 13, 15] that increase the diffusion coefficient of the enamel [18]. Among the polyphosphates, studies suggest that sodium trimetaphosphate (TMP) can reduce the solubility of hydroxyapatite mineral [19] thought to be dependent on the TMP/F ratio in the formulation [12, 17, 19, 20]. No studies, however, have evaluated the action of TMP in combination with conventional toothpastes (1100 ppm F). The TMP/F proportions have been studied at concentrations of 100 and 500 ppm F. Considering the above-mentioned synergistic effects of F and TMP on enamel demineralization when added to low-F toothpastes, it would be interesting to assess the combination of F and TMP in a conventional toothpaste (1100 ppm F), that is, if such a combination could be beneficial in reducing tooth demineralization, or if an increase in the concentration of fluoride in toothpastes would inhibit the synergistic action of TMP/F. Therefore, more studies are needed in order to increase the effectiveness of toothpastes for the reduction and prevention of polarization of dental caries among the less privileged.

Therefore, the aim of this in vitro study was to evaluate the effect of different concentrations of TMP added to fluoride toothpaste (containing 1100 ppm F) on enamel demineralization. The null hypothesis was that fluoride toothpastes (1100 ppm) with or without TMP have the same anticaries effect under in vitro conditions.

Materials and methods

Experimental design

For this study, enamel blocks (4 mm × 4 mm, n = 84) were obtained from bovine incisors kept in 2 % formalin, pH 7.0, for 30 days [21]. These blocks had their enamel surface sequentially polished, which allowed selection by determining the initial surface hardness (371 to 377 KHN; p = 0.610). The experimental design was randomized, and the blocks were divided into seven groups (n = 12 in each): toothpaste without fluoride and without TMP (placebo), toothpaste with 1100 ppm F (1100), and toothpaste with 1100 ppm F +1 % TMP (1100 1 % TMP), + 3 % TMP (1100 3 % TMP ), + 4.5 % TMP (1100 4.5 % TMP), + 6 % TMP (1100 6 % TMP), and +9 % TMP (1100 9 % TMP). The blocks were subjected to pH cycling [22] spanning 7 days and treated with the different toothpastes twice a day for 1 min. After pH cycling, the surface hardness and integrated loss of surface hardness along with the F concentration present in the enamel were determined.

Formulation, determination of pH, and concentration of F in toothpastes

The toothpastes were prepared using the following ingredients: titanium dioxide, carboxymethyl cellulose, methyl p-hydroxybenzoate sodium, saccharin, mint oil, glycerin, abrasive silica, sodium lauryl sulfate, and deionized water. TMP-containing toothpastes were prepared (Aldrich Chemistry, CAS 7785-84-4, China) at concentrations of 1, 3, 4.5, 6, and 9 % with the addition of F at a concentration of 1100 ppm in the NaF form (Merck, CAS 7681-49-4, Germany). Moreover, a toothpaste without the addition of TMP and F (placebo) and another with 1100 ppm F but no TMP were also prepared. The F concentrations and the pH of the prepared toothpastes were measured [14, 23]. The mean (SD) concentration of total (TF) and ionic (FI) fluoride (n = 3) for the placebo toothpaste were 9.5 ppm (1.1) and 9.7 ppm (0.4), respectively. For toothpastes with 1100 ppm F, the mean values (SD) between groups were 1162.0 ppm (44.1) and 1157.2 ppm (16.8), ranging between 1111.0 and 1183.9 ppm F. The mean pH among groups was 7.3 (0.3) with values ranging from 6.8 to 7.7.

pH cycling and treatment with toothpastes

The blocks were subjected to five pH cycles during 7 days, at a constant temperature of 37 °C [22]. The blocks were kept in a demineralizing solution (DE) (6 h; calcium and phosphate, both 2.0 mmol/L, in 75 mmol/L acetate buffer, pH 4.7; 0.04 μg F/mL, 2.2 mL/mm2) followed by immersion in a remineralizing solution (RE) (18 h; 1.5 mmol/L calcium; 0.9 mmol/L phosphate; 150 mmol/L KCl in 0.02 mol/L cacodylic buffer, pH 7.0; 0.05 μg F/mL, 1.1 mL/mm2). The treatment consisted of a 1-min soak under agitation in 2 mL/block of toothpaste/deionized water slurries (1:3 w/w), between immersion in the DE and RE solutions (twice a day). Deionized water rinses were performed between each step. The blocks were kept in fresh RE solution during the last 2 days of the procedure.

Analysis of enamel hardness

The surface hardness was measured using a 5114 MicroMet micro-hardness tester (Buehler, Lake Bluff, USA) with Knoop type indenter, and with a static load of 25 g for 10 s. Five indentations separated by a distance of 100 μm were made in the central region of each block (SHi). After the pH cycling, five other indentations were performed (SHf) with a distance of 100 μm from the SHi. For the cross-sectional hardness analysis, the block was sectioned at the center and one of the halves was treated with the acrylic resin and polished. A series of 14 indentations at distances of 5, 10, 15, 20, 25, 30, 40, 50, 70, 90, 110, 130, 220, and 330 μm from the external surface of the enamel was made in the central area of the block using a 5-g load for 10 s [11, 13]. The integrated area hardness (KHN × μm) of the enamel lesion was calculated using the trapezoidal rule (GraphPad Prism, version 3.02) and subtracted from the integrated area of the hardness of the sound enamel, thus yielding the integrated loss of subsurface hardness (ΔKHN) [24, 25]. The integrated area of the differential profile (i.e., the hardness at each depth from fluoride toothpastes minus hardness values from the placebo) was determined in two depth zones (5–10 and 20–130 μm) in the lesion, to yield ΔIH values [11, 25].

Analysis of the F concentration present in enamel

Blocks (2 mm × 2 mm) obtained from the remaining longitudinally sectioned blocks were attached to a mandrel and fixed to a modified microscope with an electronic digital micrometer (Starrett, São Paulo, SP, Brazil). A layer of enamel (57.5 ± 0.05 μm) was removed using a self-adhesive polishing disc of 400-grit silicon carbide (Buehler, Lake Bluff, IL, USA) fixed to the bottom of polystyrene crystal tubes (J-10 Injeplast, Paraná, Brazil) [16, 26]. The tubes, after addition of 1.0 mL HCl (1.0 mol/L), were agitated for 1 h followed by the addition of 1.0 mL NaOH (1.0 mol/L) [11, 16, 20]. For analysis of F, a previously calibrated 9409BN-specific electrode (Thermo Scientific, Beverly, MA, USA) was used. Samples and standards were buffered with TISAB II (total ionic strength adjustment buffer) at a 1:1 ratio.

Statistical analysis

For statistical analysis, we used the Sigma Plot program (version 12.0), with a level of significance set at 5 %. The SHf, ΔKHN, ΔIH, and F values were considered as variables in the enamel. The data were tested for normality (Shapiro–Wilk test) and homoscedasticity (Cochran). Data from the SHf, ΔKHN, and F values were homogenous and were subjected to one-way analysis of variance, followed by Student–Newman–Keuls’ test. The resulting ΔIH were subjected to two-way analysis of variance (toothpaste and depth zone) followed by the Student–Newman–Keuls’ test.

Results

The results of the SHf (Fig. 1a) showed that the addition of 3 % TMP produced the maximum effect in reducing demineralization (p < 0.001). Concentrations of TMP above 3 % caused a reduction in the surface hardness values (Fig. 1a). The mineral loss in depths (ΔKHN) showed a pattern similar to that of surface hardness analysis (Fig. 1b) with 3 % TMP yielding lower demineralization (p < 0.001). Compared to blocks treated with 1100 ppm F, those treated with 4.5, 6, and 9 % TMP showed smaller lesions (ΔKHN) (p < 0.001). The addition of TMP resulted in little change in enamel hardness in the outer region of the lesion (5–10 μm; Fig. 1c), and the effect of toothpastes with 1, 4.5, 6, and 9 % of TMP was similar to that of 1100 ppm F (p > 0.001). Demineralization was lower in the inner part of the lesion (20–130 μm) for toothpastes containing TMP when compared to 1100 ppm F toothpaste (p < 0.001). The addition of 3 % TMP to 1100 ppm F toothpaste (Fig. 1d) provided the highest F presence in the enamel (p < 0.001). No difference in the amount of fluoride in the enamel was observed between the toothpastes containing 1, 4.5, 6, and 9 % of TMP and 1100 ppm F toothpaste (p > 0.001).

Fig. 1
figure 1

Mean values (n = 12) from a surface hardness (SHf) after pH cycling, b integrated loss of subsurface hardness (ΔKHN), c integrated hardness of differential profiles (ΔIH) at two zones (5–10 and 20–130 μm), and d fluoride (F) present in the enamel. Distinct lowercase letters indicate the differences among groups (Student–Newman–Keuls, p < 0.001). Vertical bars denote the SD

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Discussion

The present study showed that the addition of TMP to 1100 ppm F toothpaste increases its ability to reduce mineral loss in an in vitro caries model. As observed in previous studies [16, 17], the TMP/F combination increased the capacity of toothpaste to reduce demineralization of the enamel. The reduction of mineral loss was 61 % with the addition of TMP compared to 3 % with conventional toothpaste (1100 ppm F) as determined from the hardness data on longitudinal sections (ΔKHN). These results can be explained by the adsorption of TMP to the enamel forming a protective layer against the diffusion of acid into the enamel [27]. However, when analyzing the surface hardness data, the reduction was lower at 16 %. Studies show that the action of fluoride is on the outermost regions of the carious lesion subsurface [17, 28].

As the effect of TMP is related to its ability to adsorb on the enamel, it is clear that the TMP:F molar ratio has a strong influence on the resulting anticaries effect [10, 12, 17]. The TMP/F combination affects the process of demineralization and remineralization in deeper regions of the carious lesions [11, 17, 20, 25]. In the present study, it was observed that demineralization was lower in depths (mainly at 20–130 μm) when TMP was present in the formulations, which is consistent with previous findings [11, 17, 25] (Fig. 1c). While in the outer part of the lesion (5–10 μm) only a small additional effect was produced by the presence of TMP, a greater effect was observed in the depths of the lesion. The interaction of Ca2+ and CaF+ with TMP (cross-linking) leads to reticular formation on the enamel by binding sites on the negative PO4 3− in the cyclic structure. At acidic pH, these linkages are ruptured, releasing Ca2+ and CaF+ [10, 13, 15], which can react with H2PO4 3−, leading to the formation of CaHPO4 0 and HF0. These neutral species present a diffusion coefficient a thousandfold higher than that of ionic calcium in depths in the caries lesion [18]. The addition of 1 and 3 % TMP to fluoridated toothpaste increased the concentration of fluoride in the enamel by 8 and 31 %, respectively. A previous in vitro study showed an increase of around 26 % when 3 % TMP was added to toothpaste with a lower concentration of fluoride (500 ppm).

Though it was reported that TMP and F do not compete for the same binding sites on the enamel surface [29], increasing the concentration of TMP above 3 % reduced the presence of fluoride in the enamel (Fig. 1d). Since the adsorption of polyphosphates occurs quickly [30] and can compete with the adsorption of fluoride, TMP and F must be combined in a suitable molar ratio. The synergistic effect of TMP and F appears to be dependent upon their incorporation in the dentifrices in an appropriate proportion. Combining 500 ppm F with TMP resulted in better efficacy when compared to a toothpaste with 1100 ppm F with a molar ratio of 1.2 to 3.7 [16]. With 1100 ppm F toothpaste, the equilibrium adsorption of TMP and fluoride reaches maximal effectiveness with 3 % TMP (TMP/F 1.7), unlike with 500 ppm F. Above 3 % TMP ratios (TMP/F 2.5, 3.4, and 5.1), the bonding strength of hydroxyapatite with TMP exceeds the level of reducing the anticaries synergistic effect of F. When the fluoride concentration is increased to 3000 ppm, the bonding strength with enamel is more than that with 3 % TMP with no resulting improvement in anticaries effect [17].

Conclusions

We conclude from the toothpaste formulations tested in this in vitro study that the addition of 3 % TMP to a conventional toothpaste (1100 ppm F) shows better efficacy in reducing demineralization in bovine enamel. However, the results of our study cannot be considered definitive because the pH cycling model, although simulating the development of caries under controlled conditions, has limitations. Thus, in situ remineralization protocols and in vivo studies are needed to obtain more conclusive findings. Thus, the null hypothesis was rejected.