For the prevention of caries progression and remineralization of carious lesions, the use of professional fluoride application agents is recommended, and the application of fluoride varnish, fluoride gels, and silver diamine fluoride (SDF) can be considered [1,2]. SDF was approved by the United States Food and Drug Administration in 2014 as a dentin desensitizer and has since been used to inhibit the progression of carious lesions [3]. It is particularly useful in patients whose medical history precludes the application of conventional treatments, infants in the precooperative stage, and those with severe dental fear, and can be used to prevent the progression of coronal caries lesions in both primary and permanent teeth [3,4].
SDF is composed of silver, fluoride, and ammonia ions. The silver has an antibacterial effect, the fluoride can inhibit and block the progression of caries by remineralizing the demineralized tooth structure, and ammonia serves to stabilize the solution [5,6]. However, there are disadvantages such as tooth discoloration and metallic taste due to silver, as well as an unpleasant odor and soft tissue irritation due to ammonia. These aspects should be fully explained, and patient consent should be obtained before use [7].
The iodine ions in potassium iodide (KI) can react with the excess silver ions produced during SDF application, which can reduce discoloration by forming silver iodide. Products with a two-step method of applying KI after SDF application are currently in use [8]. However, patient discomfort due to odor and additional steps for the isolation of soft tissues are the remaining problems when using SDF in clinical situations. To address these issues, Riva Star AquaTM (SDI Ltd., Bayswater, Australia), a water-based silver fluoride agent, was recently developed. Several studies have been conducted on the remineralization effects of conventional SDF agents and sodium fluoride varnishes, as well as the antimicrobial effect of silver fluoride and its ability to inhibit biofilm formation [9-11]. However, there have been few studies on the remineralization effect of water-based silver fluoride.
When applying fluoride varnish, it is recommended to apply it to a dry tooth surface rather than a wet tooth surface [12]. For SDF, it has been reported that isolation with gauze or cotton rolls and keeping the tooth surface dry before application of the agent is effective [3]. However, there is a possibility of saliva contamination before application in pediatric patients with cooperation difficulties, and studies examining the effect of saliva contamination on remineralization are still insuffi-cient.
Therefore, this study aimed to investigate the remineralization effect of water-based silver fluoride on dry and saliva-contaminated artificial caries of dentin and enamel, comparing it with SDF and sodium fluoride varnish through microhardness changes.
Riva Star AquaTM (SDI Ltd., Bayswater, Australia) with 38% silver fluoride, Riva StarTM (SDI Ltd., Bayswater, Australia) with 38% SDF, and 3MTM Fast Release Varnish (3M ESPE, St. Paul, MN, USA) with 5% sodium fluoride were used.
The specimens were prepared using sound bovine incisors without caries, discoloration, or cracks. To prepare the specimens, tissue residue was removed with a scaler and rinsed with distilled water. For dentin specimens, a low-speed diamond disk was used to remove the enamel and expose the dentin surface. For enamel specimens, a high-speed handpiece was used to cut the root. The dentin and enamel surfaces were placed with the labial surface facing upward, and the specimen was embedded using acrylic resin. The surface of the specimens was polished using 220, 600, 1,200, and 2,400 grit silicone carbamide paper (R&B Inc., Daejeon, South Korea) (Figure 1).
To induce artificial caries, a demineralization solution with a pH of 4.5 was prepared using 0.13 g of Ca(NO3)2, 250 ml of 2.2 mM KH2PO4, 250 ml of 50 mM acetic acid, and 50% NaOH [9]. For dentin specimens, artificial caries was induced by immersing the specimens in the demineralization solution for 24 hours at room temperature. For enamel specimens, the specimens were immersed in the demineralization solution for 72 hours at room temperature, with the demineralization solution changed every 24 hours.
3) Specimen classification and measuring microhardness before fluoride agent applicationEighty dentin and 80 enamel specimens with artificial caries were randomly distributed into four dentin groups and four enamel groups, with 20 specimens in each group (Table 1). The four groups were categorized as control, AF, DF, and NF, respectively. The specimens in each group were washed with distilled water for 1 minute, and then microhardness was measured using a microhardness testing machine (Mitutoyo, Kawasaki-shi, Japan). For each specimen, three distant points were chosen for measurement under loading conditions of 10 seconds of dwell time and 0.3 kgf for the dentin specimens, and 10 seconds of dwell time with 0.5 kgf for the enamel specimens. The Vickers hardness number, which was measured immediately after the induction of artificial caries, was set to VHNlesion.
Table 1 . Experimental group classification
Group | Tooth structure | Surface condition | Application method |
---|---|---|---|
Control | Dentin | Drying | Distilled water was applied for 30 seconds |
Salivary contamination | |||
Enamel | Drying | ||
Salivary contamination | |||
AF | Dentin | Drying | A thin layer of silver fluoride (Riva Star AquaTM Step 1) was applied with light tapping using a microbrush. Immediately after, KI (Riva Star AquaTM Step 2) was applied using a microbrush until the treatment surface turned clear |
Salivary contamination | |||
Enamel | Drying | ||
Salivary contamination | |||
DF | Dentin | Drying | A thin layer of SDF (Riva StarTM Step 1) was applied with light tapping using a microbrush. Immediately after, KI (Riva StarTM Step 2) was applied using a microbrush until the treatment surface turned clear |
Salivary contamination | |||
Enamel | Drying | ||
Salivary contamination | |||
NF | Dentin | Drying | A thin layer of varnish was applied in a painting motion using a supplied brush. The varnish was kept undisturbed for four hours |
Salivary contamination | |||
Enamel | Drying | ||
Salivary contamination |
Specimens in each group were treated with the fluoride agent according to the manufacturer’s instructions. The control group was treated with distilled water for 30 seconds. The AF group was treated with silver fluoride (step 1) from Riva Star AquaTM in a thin layer on the surface of the specimen using a microbrush, followed immediately by KI (step 2). In the DF group, SDF (step 1) from Riva StarTM was applied, followed by KI (step 2), and in the NF group, Fast Release Varnish was applied in a thin layer to the surface of the specimen. For half of the specimens in each group, the surface was dried using compressed air from a three-way syringe before the fluoride agent was applied. For the other half of the specimens, saliva obtained from a healthy adult with no medical history was applied to the surface using a microbrush before the fluoride agent was applied. After application of the fluoride agents, the specimens were immersed in the artificial saliva, and after 4 hours, the surfaces were rinsed with a toothbrush under running water.
The specimens were immersed in deminerealization solution for 3 hours and in artificial saliva (Xeromia Solution, Osstem Pharma Co., Seoul, South Korea) for 21 hours. The pH cycling was repeated for a total of 7 days, and the specimens were stored in a water bath at 37℃. When changing the solution, each specimen was washed with distilled water for 30 seconds, and each solution was replaced with a new solution for each trial.
5) Remineralization and measuring microhardnessThe microhardness was measured again in the same way as before. The Vickers hardness number, which was measured after pH cycling, was set as VHNpost. The difference in Vickers hardness number between after artificial caries induction and pH cycling was calculated using ΔVHN=VHNpost−VHNlesion.
Statistical analysis was conducted using SPSS 27.0 (SPSS Inc., Chicago, IL, USA). The Kruskal-Wallis test, followed by the Mann-Whitney U test, was conducted to compare the difference in microhardness according to the applied fluoride agent, with the significance level (α) set at 0.0083. The Mann-Whitney U test was performed to compare the difference in microhardness between dry and saliva-contaminated tooth surfaces, and the significance level (α) was set at 0.05.
Table 2 and Figure 2 depict the results of microhardness in dentin specimens.
Table 2 . Vickers hardness number measured after lesion formation, pH cycling, and an increase in Vickers hardness number (ΔVHN) of each dentin group
Group | Mean±standard deviation | ||||||
---|---|---|---|---|---|---|---|
Drying (n=10) | Salivary contamination (n=10) | ||||||
VHNlesion | VHNpost | ΔVHN | VHNlesion | VHNpost | ΔVHN | ||
Control | 12.12±1.86 | 13.91±1.73 | 1.79±1.22a,A | 11.56±1.61 | 14.01±2.07 | 2.45±1.23a,A | |
AF | 13.16±1.24 | 27.31±2.80 | 14.15±2.73b,A | 12.23±1.06 | 23.97±3.19 | 11.73±3.78b,A | |
DF | 11.67±2.11 | 28.30±2.34 | 16.63±3.10b,A | 11.12±1.63 | 25.85±4.18 | 14.73±4.57b,A | |
NF | 11.72±2.73 | 18.88±2.88 | 7.17±2.17c,A | 13.04±2.52 | 17.22±1.51 | 4.19±2.00a,B | |
p | 0.224 | <0.0001 | <0.0001 | 0.136 | <0.0001 | <0.0001 |
VHN: Vickers Hardness Number.
a,b,cThe lowercase letters indicate statistically significant differences between agents by Mann-Whitney U test (p<0.0083). A,BThe uppercase letters indicate statistically significant differences between surface conditions by Mann-Whitney U test (p<0.05).
p-value from Kruskal-Wallis test.
When the tooth surface was dried, the value of VHNpost increased in all four groups compared to VHNlesion. ΔVHN values were higher in the DF, AF, NF, and control groups, in that order, with no significant difference between the AF and DF groups. Both the AF and DF groups showed greater increases in microhardness compared to the NF group (p<0.0001). When compared to the control group treated with distilled water only, the AF, DF, and NF groups showed greater increases in microhardness (p<0.0001).
When the tooth surface was salivary-contaminated, the value of VHNpost increased in all four groups compared to VHNlesion. Similarly to when the tooth surface was dried, ΔVHN values were higher in the DF, AF, NF, and control groups, in that order, with no significant difference between the AF and DF groups or between the control and NF groups. The AF and DF groups showed greater increases in microhardness compared to the NF and control groups (p<0.0001).
When comparing the microhardness difference between cases where the tooth surface was dried and cases where it was salivary-contaminated, there was no significant difference in the control, AF, and DF groups. However, in the NF group, the increase in microhardness was significantly greater when the tooth surface was dried compared to when it was salivary-contaminated (p=0.004).
Table 3 and Figure 3 depict the results of microhardness in enamel specimens.
Table 3 . Vickers hardness number measured after lesion formation, pH cycling, and an increase in Vickers hardness number (ΔVHN) of each enamel group
Group | Mean±standard deviation | ||||||
---|---|---|---|---|---|---|---|
Drying (n=10) | Salivary contamination (n=10) | ||||||
VHNlesion | VHNpost | ΔVHN | VHNlesion | VHNpost | ΔVHN | ||
Control | 53.00±3.63 | 70.05±1.79 | 17.05±4.81a,A | 51.62±2.14 | 69.66±2.93 | 18.03±3.49a,A | |
AF | 52.37±2.92 | 105.81±4.51 | 53.44±5.24b,A | 52.18±1.24 | 97.95±8.34 | 45.77±8.58b,B | |
DF | 52.54±2.32 | 112.14±8.07 | 59.60±8.39b,A | 52.09±1.59 | 103.32±7.47 | 51.23±7.38b,B | |
NF | 51.53±3.20 | 91.10±4.41 | 39.57±5.77c,A | 51.62±6.11 | 81.46±5.62 | 29.83±6.33c,B | |
p | 0.942 | <0.0001 | <0.0001 | 0.808 | <0.0001 | <0.0001 |
VHN: Vickers Hardness Number.
a,b,cThe lowercase letters indicate statistically significant differences between agents by Mann-Whitney U test (p<0.0083). A,BThe uppercase letters indicate statistically significant differences between surface conditions by Mann-Whitney U test (p<0.05).
p-value from Kruskal-Wallis test.
When the tooth surface was dried, the value of VHNpost increased in all four groups compared to VHNlesion. ΔVHN values were higher in the DF, AF, NF, and control groups, in that order, with no significant difference between the AF and DF groups. Both the AF and DF groups showed greater increases in microhardness compared to the NF group (p<0.0001). When compared to the control group treated with distilled water only, the AF, DF, and NF groups showed greater increases in microhardness (p<0.0001).
When the tooth surface was salivary-contaminated, the value of VHNpost increased in all four groups compared to VHNlesion. Similarly to when the tooth surface was dried, ΔVHN values were significantly higher in the DF, AF, NF, and control groups, in that order, with no significant difference between the AF and DF groups. The AF and DF groups showed greater increases in microhardness compared to the NF group (p<0.0001). When compared to the control group, both the AF (p<0.0001) and DF (p<0.0001) groups, as well as the NF group (p=0.001), showed significantly greater increases in microhardness.
When comparing the microhardness difference between cases where the tooth surface was dried and cases where it was salivary-contaminated, there was no significant difference in the control group. However, in the AF (p=0.034), DF (p=0.041), and NF (p=0.002) groups, the increase in microhardness was significantly greater when the tooth surface was dried compared to when it was salivary-contaminated.
This study aimed to compare the remineralization effect of water-based silver fluoride, which does not contain ammonia, with that of SDF and sodium fluoride varnish using microhardness measurements in an in vitro environment. According to previous studies, SDF has demonstrated remineralization effects in both dentin and enamel caries, and sodium fluoride varnish can also inhibit the progression of dentin and enamel caries [13-15]. Therefore, after inducing artificial caries in dentin and enamel, fluoride agents were applied to investigate their remineralization effects. Additionally, considering that it may be difficult to properly dry the tooth surface before applying fluoride agents in patients with limited cooperation, each group was divided into two subgroups. One subgroup underwent tooth surface drying before fluoride application, while the other subgroup had the tooth surface contaminated with saliva before fluoride application.
In artificial dentin and enamel caries, both the AF and DF groups showed greater increases in microhardness compared to the NF and control groups. However, there was no significant difference in microhardness increase between the AF and DF groups. In artificial enamel caries, the NF group showed a greater microhardness increase compared to the control group, while in artificial dentin caries, no significant difference was observed between the NF and control groups under salivary-contaminated conditions. Furthermore, when comparing cases of tooth surface dried and salivary contaminated, significant differences were observed only in the NF group in artificial dentin caries, whereas in artificial enamel caries, significantly greater increases in microhardness were observed in the AF, DF, and NF groups when the tooth surface was dried.
Fluoride ions have a crucial role in the remineralization of carious lesions, and both the silver fluoride agent Riva Star AquaTM and the SDF agent Riva StarTM used in this study contain 38% silver fluoride. Turton et al. [16] have reported that both silver fluoride and SDF can effectively remineralize carious lesions. The silver fluoride and SDF agents used in the study both contain 0.06 g/ml of fluoride ions, whereas sodium fluoride varnish contains 0.0225 g/ml of fluoride ions. A previous study has also shown that the effective depth of penetration for fluoride, which shows how well it works, is higher in 38% SDF than in 5% sodium fluoride varnish [9]. Additionally, it has been noted that the silver ions contained in silver fluoride and SDF agents penetrate the enamel and deposit into the dentinal tubules, which can contribute to increases in microhardness [17]. Therefore, it is indicated that the difference in fluoride ion concentration and the deposition of silver ions may have influenced the increase in microhardness.
The enamel consists of 96% inorganic matter and 4% organic matter and water, while dentin is composed of 70% inorganic matter, 20% organic matter, and 10% water [18]. Enamel and dentin also exhibit differences in the progression of dental caries. Enamel caries primarily involve the dissolution of highly mineralized tissue by acids, whereas dentin caries involve both the demineralization of inorganic matter and the decomposition of organic matter [3,19]. Deposition of fluoride on the tooth surface also creates fluorapatite in the enamel, which helps the remineralization process. In dentin, remineralization occurs on the surface of hydroxyapatite crystals surrounding the organic matrix. Thus, sound dentin fibers are essential for effective remineralization to occur [20-22]. Suppressing the activity of matrix metalloproteinases (MMPs) and cathepsin, collagenases associated with the progression of dentin caries, can impede the advancement of caries [23]. Previous studies have shown that 38% SDF has a much stronger effect on inhibiting MMPs than 10% sodium fluoride. Silver and fluoride ions also inhibit the activity of collagenase, impeding their catalytic function [24,25]. Therefore, silver fluoride and SDF agents, containing silver and higher fluoride ion concentrations, are expected to effectively inhibit the activity of collagenase compared to 5% sodium fluoride varnish. Therefore, they may exhibit greater remineralization effects than sodium fluoride varnish, offering potential advantages in clinical situations for remineralizing dentin caries.
Statistically significant differences were not observed, however, the DF group showed a greater increase in microhardness compared to the AF group. Riva StarTM has a high pH ranging from 9 to 13, while Riva Star AquaTM has a neutral pH. After penetrating demineralized dentin, SDF creates an alkaline environment that promotes the phosphate groups’ and collagenous proteins’ covalent link formation [26]. Addi-tionally, the formation of calcium fluoride, generated when high concentrations of fluoride are applied, serves as a fluoride ion reservoir that can prevent caries. While calcium fluoride has low acid resistance, it transforms into fluorapatite through the remineralization process, which possesses higher acid resistance. The alkaline environment created by SDF can reduce the solubility of calcium fluoride, making it more effective as a fluoride ion reservoir [27]. Therefore, the difference in pH may influence the remineralization effects of silver fluoride and SDF, warranting further investigation in this regard.
According to previous studies, when fluoride varnish was applied to a dry enamel surface, significantly higher absorption of fluoride ions was observed compared to when moisture or saliva was present [28]. In this study, when sodium fluoride varnish was applied, higher increases in microhardness were observed when the dentin and enamel surfaces were dried compared to when the tooth surface was contaminated with saliva. Fluoride varnish, being hydrophobic, can adhere firmly to dry tooth surfaces and is hardly influenced by moisture or saliva after application [12]. However, it can be assumed that the presence of saliva on the surface before application may have a negative impact on the absorption of fluoride varnish. Therefore, more effective remineralization can be expected when the tooth surface is dried prior to application.
In enamel caries, both silver fluoride and SDF showed greater increases in microhardness when applied to the dry tooth surface compared to the surface contaminated with saliva. However, no significant difference was observed in dentin caries. Previous studies have indicated a higher absorption of fluoride ions in porous white spot lesions compared to healthy enamel, and the fluoride content of the tooth surface after applying fluoride agents was significantly higher in dentin than in enamel [29,30]. In the case of dentin, it is composed of dentinal tubules and fibers, resulting in smaller crystal sizes and a lower density compared to enamel. Additionally, due to its moist environment, ion exchange occurs more freely in dentin than in enamel, making fluoride absorption more readily than in enamel [31]. Furthermore, liquid forms of water-based silver fluoride and SDF agents exhibit superior flowability and penetrate the tooth structure more rapidly than varnish forms of fluoride [32]. Therefore, it is believed that silver fluoride and SDF agents offer various advantages for the dentin environment, including their high flowability, making them more favorable for dentin penetration even in the presence of saliva, thereby demonstrating effective remineralization.
The diamine component of SDF serves to increase the stability of the solution. Crystal et al. [6] reported that the pH of the SDF solution remained stable up to 28 days after opening, with a tendency for increased fluoride ion concentration and decreased silver ion concentration, although the changes were not significant. In silver fluoride based on water instead of ammonia, there may be differences in the stability of the solution compared to conventional SDF agent. While this study conducted experiments using bottle forms of silver fluoride and SDF immediately after opening, further study is needed to investigate changes in pH, ion concentration, and remineralization effects over time when using silver fluoride in bottle form. Additionally, this study employed pH cycling for experimentation, but it had limitations in fully replicating the oral environment. Since silver fluoride and SDF also exhibit antibacterial properties, they may be more effective in providing anticariogenic effects when used in the actual oral environ-ment. Therefore, further study on this aspect would be bene-ficial.
In this study, the remineralization effects of silver fluoride, SDF, and sodium fluoride varnish were investigated through microhardness measurement. Fluoride agents were applied to artificial dentin and enamel caries surfaces under dry and salivary-contaminated conditions. The results showed that significant increases in microhardness were observed in groups treated with silver fluoride and SDF compared to the group treated with sodium fluoride varnish. However, there was no significant difference in microhardness increase between the groups treated with silver fluoride and SDF. When comparing dry versus salivary-contaminated tooth surfaces, all groups treated with silver fluoride, SDF, and sodium fluoride varnish showed significantly greater increases in microhardness when the surface was dried in artificial enamel caries. In contrast, in artificial dentin caries, only the group treated with sodium fluoride varnish showed a significantly greater microhardness increase when the tooth surface was dried, while no significant difference was observed in the silver fluoride and SDF groups.
The presence of silver ions may have influenced the increase in microhardness, but silver fluoride, like SDF, demonstrated an increase in microhardness in artificial caries, indicating that this can lead to effective remineralization. Therefore, silver fluoride can be used for remineralization in both dentin and enamel caries, especially showing an effective remineralization effect even under salivary contamination in dentin caries.
No potential conflict of interest relevant to this article was reported.