Penetration of Silver Diamine Fluoride with and without Potassium Iodide in Carious Lesions: An In Vitro Study

1. INTRODUCTION

Early childhood caries (ECC) is a prevalent oral disease affecting the deciduous teeth of children up to six years of age [1]. This chronic condition is influenced by multiple factors, including diet, the oral microbiome, dental hygiene, and social determinants of health [2]. Untreated ECC can result in malocclusion, abscesses, and pain [3]. Severe ECC (S-ECC) may further impact a child’s oral health, nutrition, and growth [4]. Prevention strategies focus on reducing the transmission of Streptococcus species, limiting dietary sugars, applying topical fluoride, and conducting early dental examinations [5]. Providing conventional restorative treatments to young children with severe dental issues, including those with special needs, remains challenging even when dental services are accessible [6].

Silver diamine fluoride (SDF), a formulation containing fluoride, ionic silver, ammonia, and water, was approved by the United States Food and Drug Administration in 2014 as an anti-hypersensitivity agent [7]. SDF is a cost-effective and user-friendly topical treatment that has gained widespread use due to its effectiveness in arresting caries [8]. It slows caries progression in both deciduous and permanent teeth, reduces root caries in elderly patients, and alleviates tooth sensitivity by occluding dentinal tubules. The strong antibacterial action of silver ions in SDF significantly inhibits oral bacteria, particularly Streptococcus mutans [9-12]. The combination of silver and fluoride exerts a synergistic effect in halting caries progression, distinguishing SDF from other fluoride-containing products [13].

A notable drawback of SDF is tooth discoloration. Studies have shown that decayed enamel and dentin may turn dark brown or black following SDF application [14]. Higher rates of demineralization increase silver ion absorption, intensifying the color contrast between damaged and healthy tissues [15]. This staining is persistent and can only be removed by physical methods [16]. The addition of potassium iodide (KI) to SDF has been proposed as a solution to reduce discoloration. KI reacts with excess silver ions in SDF to form silver iodide, a light-sensitive, insoluble precipitate that prevents black staining on teeth [17]. SDF/KI products, such as those from Dental KEDO, are designed to reduce SDF-related discoloration while maintaining anti-caries efficacy [18-20].

Previous research on KI has explored its effects on dentin, oral bacteria, and restoration bond strength in SDF/KI-treated teeth [21]. However, further studies are required to fully assess its efficacy in arresting dental caries compared to SDF alone. A systematic review by Mungur et al. reported conflicting evidence on the effectiveness of SDF combined with KI, suggesting that KI may limit SDF’s caries-arresting ability, although these differences may not be directly attributable to KI addition [22]. Most existing studies have focused on healthy dental blocks or animal models, providing limited information on the interactions of SDF and KI in carious lesions of primary teeth. Evidence from recent systematic reviews indicates that few studies have examined the penetration depth of SDF with KI in untreated carious teeth, as most studies involved prior caries removal or cavity preparation [23].

Therefore, this study aimed to evaluate the penetration of SDF combined with KI in caries lesions without prior caries removal.

2. MATERIALS AND METHODS

3. RESULTS

Table 1 presents the mean fluoride, silver, and iodide ion weight percentage concentrations (wt%) in affected dentin for all study groups. Fluoride ion penetration showed a statistically significant difference among groups (P = 0.009). The SDF group had the highest fluoride concentration (1.37±0.89 wt%; 95% CI: 0.63 to 2.11), followed by SDF/KI (0.47±0.35 wt%; 95% CI: 0.15 to 0.79), and the control group (0.28±0.24 wt%; 95% CI: –0.03 to 0.58). Silver (P = 0.097) and iodine (P = 0.066) ions did not show significant between-group differences. However, higher mean values were observed in SDF and SDF/KI compared to the control. For silver: SDF (4.32±0.70; 95% CI: 3.72–4.91), SDF/KI (2.90±0.48; 95% CI: 2.51–3.31), and control (0.04±0.05; 95% CI: –0.04–0.13). For iodine: SDF/KI (3.72±2.57; 95% CI: 1.57–5.87), SDF (1.10±0.34; 95% CI: 0.84–1.51), and control (1.58±0.82; 95% CI: 0.84–2.35).

Figure 1 presents a bar graph depicting the mean penetration levels of fluoride, silver, and iodide ions in the control, SDF, and SDF/KI groups.

Table 2 shows the Games-Howell post hoc test results for pairwise comparisons of the study groups for the fluoride ion penetration variable. The results revealed that the SDF group showed significantly higher fluoride ion penetration than the control group (P = 0.024). No significant differences were observed between the other study groups (P > 0.05).

This increased fluoride ion penetration in the SDF group is clinically relevant, as it suggests a greater potential for remineralization and caries arrest in the deeper dentin layers. The SDF/KI group demonstrated a lower surface fluoride concentration than SDF alone but still showed improved fluoride diffusion over the control, indicating a therapeutic benefit with reduced esthetic concern.

Table 3 presents the mean elemental concentrations (wt%) across the study groups. Fluoride ion levels were also elevated in the SDF (1.37 ± 0.89) and SDF/KI (0.47 ± 0.35) groups relative to the control (0.28 ± 0.24). Silver ion concentrations were higher in the SDF (4.32 ± 3.70) and SDF/KI (2.90 ± 2.89) groups compared to the control group (0.04 ± 0.05). Iodine ion concentrations in the SDF (1.80 ± 0.34) and SDF/KI (3.72 ± 2.57) groups exceeded those of the control (1.58 ± 0.62). Calcium (Ca), carbon (C), and oxygen (O) had the highest concentrations among all groups.

Figures 2-4 shows SEM and EDX chemical analysis images of sectioned teeth from the SDF, SDF/KI, and control groups, highlighting both infected and non-infected dentin. The EDX spectral analysis confirmed that the primary elements detected in affected dentin treated with SDF and SDF/KI were calcium (Ca), phosphorus (P), iodine (I), oxygen (O), carbon (C), and silver (Ag). Fluoride (F), magnesium (Mg), and sodium (Na) were also detected. Silver ions were observed at 0-3 keV and around 22 keV, distinct from other ions. The highest peaks were for Ca at 3.6 keV and P at 2 keV. In the control group, levels of Ca, P, I, C, and O were similar to those in the treated groups; however, Ag and F levels were lower. The highest peaks in the control group were linked to Ca and P.

Figure 5a-c displays the elemental mapping of the same tooth sections. In the SDF group, increased accumulation and movement of silver (Ag), iodide (I), and fluoride (F) ions were observed in the superficial carious lesion, with fluoride showing the greatest density difference between the superficial and deep areas. In the SDF/KI group, silver and iodine ions accumulated more in the superficial area, while fluoride penetrated deeper compared to the control group, although its surface concentration was lower than in the SDF group. Fluoride distribution appeared more evenly dispersed throughout the lesion depth in this group. The control group exhibited lower overall levels of silver and fluoride ions, with a nearly homogeneous distribution of all three ions throughout the tooth structure.

4. DISCUSSION

The caries-arresting potential of silver diamine fluoride (SDF) is attributed to its dual action: antimicrobial silver ions inhibit cariogenic bacteria, while fluoride ions promote remineralization of tooth structures. This dual mechanism allows SDF to serve as both a preventive and therapeutic agent, particularly prior to the initiation of restorative procedures [29]. Studies confirm its safety, showing no toxicity or pulp damage following indirect application [30]. Moreover, the simplicity of the SDF application provides a significant advantage over conventional operative treatments, as it can be performed in diverse settings without advanced equipment [31]. A notable drawback is black discoloration caused by silver phosphate (Ag₃PO₄) deposition [32]. Applying potassium iodide (KI) after SDF reduces this effect by forming a yellow iodide precipitate, preventing silver phosphate formation [33]. Previous research consistently demonstrates that 38% SDF is highly effective in managing and arresting dental caries in primary dentition [34]; hence, this concentration was used in the present study.

In the current study, the concentrations of silver, fluoride, and iodine ions were higher in the SDF and SDF/KI groups compared to the control. Water-soluble SDF readily penetrated carious lesions due to their high permeability and moisture content. The loss of mineral content in peritubular and intertubular dentin further facilitated transdentinal and intradentinal diffusion of SDF [35]. Additionally, the low molecular weight of SDF enhances deeper penetration [36].

Unlike the SDF/KI and control groups, which exhibited relatively homogeneous ion distribution, the SDF group showed reduced fluoride diffusion from the surface toward the lesion depth. The two-step SDF/KI application may explain this: the initial fluoride-rich SDF layer is followed by KI application, which reduces the superficial fluoride concentration without affecting deeper penetration [28]. Thus, KI does not negatively impact the remineralizing and caries-arresting properties of SDF but primarily modifies the surface fluoride layer. These findings align with a systematic review by Papagatsiou et al., which reported that KI addition does not compromise—or only slightly reduces-the efficacy of SDF in arresting caries [37].

The reduction of surface fluoride may have clinical advantages, as lower superficial fluoride could reduce interference with adhesive bonding, potentially improving the bond strength of subsequent restorative materials to SDF-treated dentin and reducing microleakage. This effect mirrors benefits observed from washing, etching, or polishing after SDF application [38, 39]. Consequently, KI use may enhance compatibility with restorative procedures, including composite resin, resin-modified glass ionomer cement, and other adhesive systems. Manuschai et al. demonstrated that adding KI does not significantly affect the antibacterial efficacy of SDF against mixed-species biofilms on human root dentin [40]. Similarly, Kaur et al. found that SDF penetration is more dependent on the natural dentin structure than on the presence of KI, indicating that KI does not compromise SDF’s protective properties [41]. Manuschai et al. also reported that sclerotic dentin in deeper lesions limits SDF penetration, suggesting that lesion depth and microstructure significantly influence ion diffusion [35].

Hydration is another critical factor for ion transport, as diffusion primarily occurs through aqueous pathways [42]. Since extracted teeth were used in this study, dentinal tubules were likely dehydrated, and pulpal pressure was absent, potentially reducing ion diffusion compared to in vivo conditions [42, 43].

Elemental mapping showed greater silver ion penetration in superficial areas, likely due to the exposure of reversibly denatured collagen, which facilitates silver precipitation via functional groups such as sulfur and nitrogen [15, 35, 44]. Zaneldin et al. reported that silver ions penetrate hard tooth tissues more effectively in areas of advanced caries due to increased collagen exposure [30]. Rajakumari et al. noted that silver ions maintain sustained biocidal activity even after killing bacteria, as dead bacteria serve as reservoirs for silver release [45]. Bhatt et al. observed that light exposure catalyzes silver precipitation in superficial infected dentin layers, enhancing silver ion deposition in carious lesions [46].

In this study, fluoride ion penetration was significantly higher in the SDF group than in the control, confirming SDF’s effect on enhancing remineralization by forming stable compounds such as fluoro-hydroxyapatite or calcium fluoride-like particles, which act as fluoride reservoirs [36]. Surface analysis by Samani et al. similarly revealed higher silver and fluoride ion content in SDF-treated samples compared to controls [13]. Rogalnikovaitė et al. reported that SDF interaction with dental hydroxyapatite forms calcium fluoride, which is soluble in saliva and serves as a fluoride reservoir [47]. Conversely, Mulder et al. demonstrated that when SDF is combined with KI or glutathione, some silver ions are absorbed, allowing fluoride to act more efficiently over time [28]. The lack of significant difference in fluoride penetration between SDF and SDF/KI groups in this study may relate to the short experimental duration and the low stability of calcium fluoride [35]. Variations in KI concentration may also influence these outcomes [23].

No significant differences were observed in silver ion penetration between SDF and SDF/KI groups, indicating that KI addition to reduce discoloration does not substantially affect silver diffusion. This may be due to the absence of surface etching in the study, as the smear layer can limit ion deposition. Etching increases surface adhesion for KI, potentially reducing silver ion release [31].

These findings have important implications in pediatric dentistry, where non-invasive and efficient treatments are critical. SDF, with or without KI, provides a child-friendly option for caries management, particularly in young or uncooperative patients. Khan et al. found that SDF/KI improved the bond strength of glass ionomer cement to primary dentin without compromising composite resin bonding, even after thermal aging [48]. Ballikaya et al. demonstrated the long-term efficacy of SDF and SMART techniques in managing molar-incisor hypomineralization, reducing dentinal hypersensitivity, and stabilizing carious lesions [49]. SDF application before stainless steel crown placement, as in the Hall technique, may further optimize outcomes in carious primary molars [48].

While the study provides valuable insights into SDF and KI interactions in carious lesions, further research is warranted. Future studies should evaluate bond strength, caries-arresting efficacy, and antibacterial effects of SDF and SDF/KI, considering variables such as rinsing duration, etching, light curing, surface conditioning, and time-dependent penetration. Both animal and clinical studies are necessary to validate these findings and assess their clinical relevance.

Limitations of this study include the difficulty in obtaining suitable tooth samples and the inability to replicate in vivo oral conditions in a laboratory setting. Therefore, caution is advised when extrapolating these results to clinical practice, and further clinical investigations are needed.

CONCLUSION

Silver diamine fluoride (SDF) markedly enhanced fluoride ion penetration in carious dental structures. The addition of potassium iodide (KI) decreased surface fluoride concentrations compared to SDF alone, without affecting the levels of other ions or the deeper diffusion of SDF. This reduction in surface fluoride may improve the adhesion of restorative materials by minimizing interference with bonding agents, potentially enhancing bond strength and the longevity of restorations. Clinically, the combined use of KI with SDF is particularly advantageous in esthetically sensitive areas, as it reduces tooth discoloration while maintaining the caries-arresting efficacy of SDF.

REFERENCES