Introduction
Dental caries is a persistent global oral health challenge, and a meta-analysis of studies from 1995 to 2019 revealed high prevalence rates of 53.8% and 46.2% in the permanent and primary teeth, respectively [1]. In addition to discomfort and toothache, untreated caries can cause oral mucosal diseases [2], infections [3], and difficulties in eating and sleeping [4], which adversely impact the quality of life of children and their caregivers [5]. Caries in the primary teeth are strongly correlated with those in their successors, leading to developmental abnormalities and long-term oral health issues [6]. Effective management of caries in the primary teeth is crucial for protecting the health of the permanent teeth; thus, early detection and intervention are important. This aligns with the principles of minimal intervention dentistry, which focuses on the early recognition of caries and minimally invasive remineralization to halt its progression while improving aesthetics and function [7,8].
Various therapeutic agents are used to manage caries. Among these, silver diamine fluoride (SDF) is a reliable option for inhibiting caries progression without tooth removal [9,10] and for preventing new lesions [11]. SDF is an ammonia-stabilized colorless alkaline solution containing fluoride ions for remineralization and silver ions for antibacterial properties. It is commonly used at a concentration of 38% with 44,800 ppm fluoride [12]. Recently, interest in SDF has surged globally owing to its advantages [13], such as minimal invasiveness, safety, ease of application, efficiency, and low cost [14,15]. SDF is particularly useful for patients at high risk for caries, with limited cooperation, or with systemic conditions that make conventional caries treatment challenging. Additionally, it can be effectively applied in cases where complete caries treatment cannot be accomplished in a single visit and in challenging areas such as the occlusal surfaces of incompletely erupted molars or secondary caries at the crown margins [16]. However, despite its excellent clinical efficacy, SDF has a major drawback: the deposition of silver compounds on the tooth surface causes discoloration of the carious structure [17]. Other issues include its high fluidity, which limits the contact time with the lesion, and the reduction in fluoride concentration due to saliva dilution [12].
To overcome these limitations and clinical barriers, efforts to standardize the application protocol of SDF to optimize results are ongoing [18]. The American Academy of Pediatric Dentistry (AAPD) guidelines recommend the use of SDF for at least one minute, followed by drying and isolation for a minimum of three minutes. If cooperation issues exist, shorter application times may be allowed; however, careful monitoring and reapplication are necessary [19]. Nevertheless, even the minimum recommended application time may not always be feasible in clinical settings. The possible outcomes of integrating SDF with light curing as a novel approach are currently attracting greater interest [18]. In vitro studies on dentin caries reported that, in comparison to using SDF alone, light curing after SDF application increased dentin microhardness [20,21] and shortened the application time of SDF [22]. Additionally, potassium iodide (KI) has been used to address the discoloration issue. KI reduces discoloration by precipitating excess silver ions as silver iodide (AgI) when added right after SDF [17,23].
Regarding enamel caries, SDF has been shown to promote remineralization owing to its high fluoride concentration and the formation of silver-containing hydroxyapatite, in addition to reducing bacterial adhesion and lowering cytotoxicity [12]. Dental caries begins when subclinical enamel lesions are caused by mineral loss from acidic bacterial byproducts. If preventive measures are not implemented, caries progress to clinical manifestations, eventually reaching the dentin and forming definite cavities. According to the latest recommendations from oral health institutes, early detection and management of lesions are recommended to minimize tissue loss and invasive caries treatment [23]. Thus, the remineralization ability of enamel caries warrants further investigation. However, most studies have focused on the management of dentin caries, and few studies have explored the use of SDF in conjunction with light curing for enamel caries. Moreover, although KI influences the remineralization with SDF, studies on the combined ef-fects of KI and light curing are insufficient.
Therefore, this study aimed to assess the effect of light curing following SDF and KI application on remineralization of artificially induced enamel caries. In addition, the effect of a shortened application time of 10 seconds followed by light curing was analyzed and compared with that of the recommended one-minute application.
Materials and Methods
This study was approved by the Institutional Review Board of the Wonkwang University Dental Hospital (WKDIRB202404-02). Fig. 1 presents the study flowchart.
1. Sample size calculation and specimen preparation
The sample size for this study was determined using G*Power software (version 3.1.9.7, Heinrich Heine University Düsseldorf, Düsseldorf, Germany). For Vickers microhardness measurements, the sample size was based on a pilot study conducted by the authors prior to this study and a previously published study on dentin caries [22]. A one-way ANOVA statistical test indicated that at least six specimens per group were required (α = 0.05, β = 0.20). To account for potential errors in specimen preparation and data analysis, the sample size was increased to 10 specimens per group. Additionally, five specimens per group were allocated for field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) analysis, as this was the minimum number required for the Kruskal-Wallis test to assess silver ion precipitation among groups. Therefore, a total of 64 specimens were used in this study, including 15 per group for SDF treatment and two each for observing the polished sound enamel and caries-induced enamel surfaces.
Sixty-four specimens were prepared from 32 primary molars with sound crowns that were free of caries, structural defects, or restorative materials. The teeth were extracted either because of imminent physiological shedding or for the treatment of ectopic eruption or eruption disorders of the successor teeth. The crowns of the primary molars were sectioned along the buccolingual axis and embedded in acrylic resin using 25.0 mm diameter cylindrical molds (Fig. 2). The surfaces of the specimens were sequentially polished using silicon carbide paper (R&B Inc., Daejeon, Korea) with grit sizes of 400, 800, 1200, and 2000. Only teeth with enamel microhardness values between 270 - 340 Vickers microhardness numbers (VHNpre), measured using a microhardness tester (MXT70, Matsuzawa Co., Ltd., Akita, Japan), were selected for further experimentation. Of the 64 specimens, two were used for observation of the polished enamel surface using FE-SEM (S-4800, Hitachi, Ltd., Tokyo, Japan). The remaining specimens were submerged in deionized distilled water at 4°C until artificial caries induction. The deionized distilled water was replaced daily.
2. Artificial caries induction
To induce artificial caries, a demineralization solution with a pH of 4.4 was prepared using 50 mM acetate, 2.2 mM, and 2.2 mM, following the protocol described in a previous study [15]. After excluding two specimens for surface observation, 62 specimens were submerged in the demineralization solution and maintained in a incubator (VS-9160C, Vision Scientific Co., Ltd., Daejeon, Korea) at 5% and 37℃ for 96 hours to artificially induce enamel caries.
3. Pre-treatment measurement
Demineralized specimens were washed with distilled water for one minute, and the carious enamel surfaces of two specimens were examined at ×8,000 and ×30,000 magnifications using FE-SEM. Microhardness of the carious enamel (VHNlesion) in the remaining 60 specimens was measured. Three points on each specimen were measured using the microhardness tester under a 0.2 kgf force with a 10-second dwell time, and the average values were calculated.
4. SDF application and pH cycling
Sixty specimens were randomly allocated to four groups. SDF (38%; Riva Star Step 1, SDI Ltd., Bayswater, Australia), KI (Riva Star Step 2, SDI Ltd.), and an LED light-curing unit (VALO™ Cordless, Ultradent, South Jordan, UT, USA) were used. The specimens were air-dried for 10 seconds using a three-way syringe, and SDF (1.0 μ L) and KI (2.0 μL) were applied using a disposable microbrush according to the protocol described in Table 1. SDF was continuously rubbed for either 60 seconds or 10 seconds, depending on the experimental group. KI was immediately applied after SDF application and continu-ously rubbed for 10 seconds until the initially creamy white appearance turned transparent, following the manufacturer’s instructions.
The SDF application times were determined based on a previous study [22] that compared 10-second and 60-second applications on dentin carious lesions, with and without light curing. Following this approach, this study aimed to assess whether a 10-second application with light curing could achieve enamel remineralization comparable to the conventional 60-second application. During LED curing, the curing light tip was positioned at a distance of 4.0 mm from the specimen surface.
After reagent application and curing, the specimens were air-dried for 10 seconds using a three-way syringe and washed with distilled water for 10 seconds according to the manufacturer’s recommendations.
Based on the protocol described in a previous study [15], pH cycling was performed to analyze the effect of each application method on the remineralization of artificially induced enamel caries. A demineralization solution (pH 5.0) containing 50 mM acetate, 1.5 mM, and 0.9 mM and a remineralization solution (pH 7.0) containing 150 mM KCl, 20 mM HEPES, 1.5 mM, and 0.9 mM were used. The specimens were stored in a incubator at 5% and 37°C and immersed in the demineralization and remineralization solutions for 3 hours and 21 hours, respectively. This cycle was repeated for 8 days, with the solutions replaced daily. Specimens were washed with distilled water between solution transitions.
5. Post-treatment measurement
Post-treatment FE-SEM imaging and microhardness measurements (VHNpost) were performed using the same method as for the pre-treatment evaluations. Five specimens were randomly selected from each group for FE-SEM imaging, and additional elemental component analysis was performed using EDS (S-4800, Hitachi, Ltd., Tokyo, Japan). The remaining 10 specimens from each group underwent microhardness measurement. The difference in microhardness values before and after SDF application was calculated using the formula: ΔVHN = VHNpost ‒ VHNlesion.
6. Statistical analysis
IBM SPSS Statistics (version 29.0; IBM Corp., Armonk, NY, USA) was used for all statistical analyses. The Shapiro-Wilk test was used to evaluate the normality of distribution of the measured microhardness values. One-way ANOVA test was used to evaluate the differences among the four groups, followed by post hoc analysis using Tukey’s honestly significant difference test. The mean silver ion precipitation was compared using the Kruskal-Wallis test. The Mann-Whitney U test was used for post hoc analysis. Statistical significance was set at α = 0.05.
Results
1. Microhardness changes
Table 2 lists the Vickers microhardness number (VHN) measured at each stage. The differences in VHNpre and VHNlesion were not significantly different between the groups (p= 0.170 and p= 0.421, respectively). In contrast, significant differences were observed among the groups in both the VHNpost values measured after pH cycling and the ΔVHN values, which represent the difference between post-treatment and pre-treatment microhardness (both p < 0.0001).
The ΔVHN results are presented in Fig. 3, and the results of the post hoc multiple comparisons of ΔVHN among the groups are presented in Table 3. Post hoc analysis revealed that Group IV, which underwent SDF application for 10 seconds followed by KI application and light curing, showed the greatest increase in microhardness compared to the other groups. Group III, which received SDF application for 10 seconds followed by light curing, exhibited the second-highest increase, and ΔVHN was significantly different between Groups III and IV (p < 0.0001). Groups I and II showed significantly smaller increases in microhardness than the light-cured groups (p < 0.0001), with Group II, where SDF was applied for only 10 seconds, displaying the lowest increase. However, ΔVHN was not significantly different between Groups I and II (p= 0.316).
2. Surface structure and composition analysis
Fig. 4 shows the images of polished sound enamel and enamel with artificially induced caries at ×8,000 and × 30,000 magnifications. The demineralized enamel exhibited a rough and irregular surface with numerous porous areas. Fig. 5 shows the microstructures of the enamel surfaces in each group following SDF treatment and pH cycling. In Group I, hydroxyapatite crystals and spherical particles were observed on the enamel surface, with a reduction in surface irregularities compared to the demineralized enamel. Group II showed findings similar to those in Group I; however, the porous areas were more prominent. In Group III, the surface appeared denser with numerous distinct, angular, and bright particles. Similarly, Group IV exhibited a denser surface with filled porous areas and angular crystals. Notably, Group IV contained dense, non-uniform white aggregates.
EDS analysis revealed silver ions on the enamel surface in all groups, indicating the deposition of silver compounds. Iodine was detected in Group IV, where KI was applied, suggesting the formation of AgI (Fig. 6). Table 4 shows the mean rank and mean ± SD of silver ion precipitation for each group. The Kruskal-Wallis test revealed significant differences among the groups (p= 0.002). Post hoc tests using Bonferroni’s method and Mann-Whitney U tests further confirmed that Groups III and IV had significantly higher silver ion precipitation than Groups I and II (p= 0.008), whereas no significant differences were observed between Groups I and II (p= 0.222) and between Groups III and IV (p= 0.421).
Discussion
This study demonstrated that the combination of shortened SDF application time, KI application, and light curing significantly improved remineralization of enamel caries. Group IV (SDF + KI + light curing) exhibited the greatest increase in microhardness, followed by Group III (SDF + light curing). Furthermore, SEM-EDS analysis confirmed a higher concentration of silver particles in the light-cured groups and revealed iodine peaks in Group IV, indicating the formation of AgI complexes on the enamel surface. These findings highlight the synergistic role of KI and light curing in enhancing remineralization, and their potential to improve clinical outcomes in the management of enamel caries.
Light curing improved the ability of SDF to remineralize enamel caries, despite the reduced application time. This is consistent with the findings in previous studies that light acts as a catalyst, accelerating the reduction of silver ions in solution and facilitating their concentration in the infected dentin [24]. Additionally, the caries-arresting effect of SDF has been attributed to silver complexes filling the voids created by caries and increasing the density of the residual organic material, rather than fluoridemediated remineralization [25]. The results of this study were interpreted while considering the differences in the mechanisms of SDF action on enamel and dentin. Light curing after SDF application induces greater silver ion deposition in the infected dentin, leading to increased microhardness. Furthermore, the high silver concentration in the infected dentin may limit its penetration into the deeper layers of sound dentin [21]. This suggests that although SDF alone can penetrate all layers (infected, affected, and sound dentin) of dentin, light curing primarily affects the superficial layer comprising the infected dentin, resulting in more effective remineralization at this specific layer. In the case of enamel, because of its higher density and lower permeability than dentin and its ability to absorb most of the light, the effects of light curing are likely to be more concentrated on the surface. The findings in this study support this hypothesis, because the groups with light curing demonstrated a significant increase in the microhardness of the enamel surface, which indirectly indicates the surface-focused effect of light curing on the enamel.
The exact mechanism by which LED curing light affects SDF’s remineralization effect remains unclear. However, based on previous studies, several hypotheses can be considered. One possible explanation is the “diffusion enhancement hypothesis”. The formation of carious lesions is fundamentally driven by the diffusion of acids into enamel and dentin and the outward diffusion of calcium and phosphate [26]. One study found that with 60 seconds of SDF application, silver ions penetrated an average of 590 μm into the dentin lesion. Considering the diffusion coefficient of silver ions, with a 10-second SDF application, the expected penetration rate was 30%, with an actual observed penetration of 26%. However, when a 10-second SDF application was followed by 20 seconds of LED light curing, the penetration rate increased to 94%, surpassing the predicted 50% [27]. While the precise mechanism remains unclear, it is hypothesized that silver ions and silver nanoparticles strongly absorb blue light, increasing their kinetic energy and facilitating enhanced diffusion into the lesion [28]. Additionally, the photothermal effect generated by LED curing may contribute to increased ion mobility, promoting deeper penetration and precipitation of silver ions [27].
Some studies have also indicated that natural light promotes the reduction of silver solutions, facilitating the generation of silver ions and nanoparticles. Since dental curing lights function as initiators, they may accelerate the reduction of silver ions and promote their precipitation in infected dentin, reducing the time available for penetration and leading to a more concentrated effect on the lesion [24].
In summary, while LED curing does not induce photopolymerization in SDF, it appears to facilitate silver ion reduction and enhance their precipitation within carious lesions, primarily through increased diffusion, photothermal effects, and catalytic reduction mechanisms. Further research is needed to clarify these interactions and their long-term clinical implications.
In addition, when SDF is applied to the enamel, silver ions react with hydroxyapatite to form silver compounds, and exposure to light enhances the formation of metallic silver nanoparticles [29]. Silver nanoparticles from SDF exhibit potent antimicrobial effects, inhibit carbohydrate metabolism in plaque, and alter the balance of plaque bacteria. These effects are particularly effective against mutans streptococci, resulting in suppression of biofilm formation and promotion of remineralization of demineralized enamel and dentin [30].
Group IV demonstrated the highest increase in microhardness among all groups, emphasizing the synergistic role of KI and light curing in enamel remineralization. The effect of KI on the remineralization efficacy of SDF has varied in previous studies, depending on the objectives and methodologies. Some studies reported no significant differences in enamel remineralization between SDF + KI and SDF alone [31,32]. In contrast, other studies have demonstrated that the enamel surface treated with SDF + KI was more uniform, with a significantly higher microhardness compared to that treated with SDF alone or SDF + 5% sodium fluoride varnish [33]. One hypothesis suggests that when silver ions from SDF bind weakly to the enamel surface, residual silver ions dissolved during pH cycling may interfere with fluoride-mediated remineralization by inhibiting the effective precipitation of these ions [34]. This finding suggests that KI reacts with SDF to remove residual silver ions that could interfere with fluoride-mediated remineralization and that light curing rapidly precipitates silver ions, thereby promoting effective remineralization. This synergistic effect may occur because KI modulates the reaction of silver ions, thereby enhancing the efficiency of light curing.
SEM-EDS analysis confirmed the formation of AgI, which enhanced both the antibacterial properties of silver and the remineralization efficacy, in the KI-treated group. Previous studies have reported that the deposition of AgI can influence the mineral density and increase the microhardness of the enamel and dentin [35]. Furthermore, this protective layer prevents further loss of calcium and phosphate from demineralized enamel [29]. These findings support the possibility that concurrent KI application enhances enamel remineralization more than that with SDF treatment alone owing to AgI formation. However, additional research is necessary to determine whether the antimicrobial and remineralization effects of AgI can fully explain the positive changes in the tooth surface. Although AgI is chemically stable, clear data regarding its stability and longevity in the complex oral environment are lacking. Moreover, further investigation is required to understand the mechanism by which the combination of KI and light curing synergistically enhances AgI formation and silver ion reactions. Such studies could provide a more comprehensive understanding of the long-term effects and clinical applicability of AgI.
The 10-second SDF application in Group II induced limited remineralization compared to that with the conventional one-minute application in Group I; however, the difference was not statistically significant. This suggests that, as recommended by the AAPD [19], careful follow-up may be necessary for patients who are treated using a shorter application time owing to cooperation issues. However, when light curing was incorporated (Group III), the remineralization efficacy significantly improved, despite the reduced application time. Group IV, in which light curing was combined with KI application, demonstrated the greatest efficacy, suggesting that protocol modifications can compensate for shorter application times. These results are particularly important for enamel caries for which patient compliance and clinical feasibility are often challenging. Enamel lesions require early intervention to prevent progression to dentin caries, and the ability to achieve effective remineralization with a shorter application time can facilitate treatment in uncooperative patients, especially children. In addition, for early caries on proximal surfaces, which are difficult to access and challenging to treat, this modified SDF application method is expected to effectively inhibit caries progression and facilitate management. This approach also addresses aesthetic concerns associated with SDF, improving its acceptability as a minimally invasive strategy for managing enamel caries.
Despite the promising findings, this study had several limitations. First, the in vitro conditions used did not ful-ly replicate the complex oral environment. Factors such as salivary flow, bacterial biofilms, and dietary influences may affect the efficacy of SDF, KI, and light curing. Further in vivo studies are necessary to validate these results under clinical conditions. Second, although KI reduced discoloration and enhanced remineralization, the extent and clinical relevance of its aesthetic benefits were not quantitatively assessed. Considering the high aesthetic demands in treating enamel caries, future research should focus on measuring discoloration and comparing aesthetic outcomes across different protocols.
Conclusion
This study demonstrated that SDF combined with light curing has positive effects on enamel remineralization and caries inhibition. Additionally, the concurrent application of KI and light curing resulted in a notable increase in enamel microhardness, underscoring its potential as a minimally invasive strategy for managing enamel caries. The incorporation of light curing allows for treatment completion in a shorter duration compared to conventional SDF application, offering practical advantages in clinical settings.
Although KI is intended to mitigate the discoloration associated with SDF, this study did not quantitatively assess the extent of discoloration reduction. Therefore, further research is needed to objectively evaluate the aesthetic impact of KI when combined with SDF and light curing. Nonetheless, this approach holds significant promise for effectively managing early caries in patients with limited cooperation and challenging clinical conditions and offers advantages as a minimally invasive and preventive treatment strategy.
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