Introduction

Dental caries has remained a significant public health issue globally, with a prevalence of up to 90% in schoolchildren and adults, especially in disadvantaged populations [1]. Among the non-invasive strategies for managing caries, silver diamine fluoride (SDF) treatment has recently gained popularity due to several advantages, such as antimicrobial properties, a remineralization effect, cost-effectiveness, and accessibility for children and elders [2–4]. Studies have documented the efficacy of SDF in arresting caries on both primary [5] and permanent teeth in clinical settings [6], as well as its capacity to enhance the microhardness and mineral density of carious lesions in- vitro experiments [7–9].

Results demonstrated that 38% SDF is the most effective concentration in arresting caries in primary teeth [10, 11] with a biannual application demonstrating higher caries-arrest rates than an annual application [12, 13]. However, several factors can influence the efficacy of SDF, including the location of the tooth, cavity size, baseline caries experience, oral health-related behaviors, fluoride exposure, the ability to cooperate, and the application procedure [14]. Despite efforts to standardize the protocol for SDF application, the recommended protocols relied on experts’ opinions, thus varying in practice [4]. Several studies have proposed that 60 seconds is the recommended duration for SDF applications; however, this may be too long for younger children, those with special needs, and older adults [4, 15].

The efficacy of SDF in managing dental caries in clinical settings has prompted dental clinicians and researchers to develop strategies to enhance its utilization [16]. In reference to a study conducted by Fung et al. [17], Crystal et al. suggested that the higher caries-arrest rates in anterior teeth compared to posterior teeth might be attributable to more natural light exposure, leading to more active silver precipitation [18]. Hence, our research question aims to investigate whether the combination of light curing and silver diamine fluoride (SDF) application enhances the effectiveness of SDF in treating carious lesions in primary teeth compared to SDF alone, by potentially accelerating the rate of silver precipitation, shortening application time, and achieving a more rapid arrest of caries. [15]. Recent ex vivo studies have highlighted that the combination of SDF and light curing application results in a considerable rise in the dentine microhardness, mineral density, and silver ion precipitation rates compared to SDF alone [15, 19, 20]. Although studies have demonstrated the potential benefits of SDF application combined with light curing on carious lesions, there remains a significant gap in the literature regarding its mechanism of action and overall effectiveness. Therefore, this systematic review aims to evaluate the potential benefits and underlying mechanisms of combining SDF with light curing, based on available studies. As SDF has been included into the Model List of Essential Medicines of the World Health Organization in 2022, alongside the growing adoption and interest in SDF [21], evidence-based protocols for SDF treatment are pivotal to ensure optimal clinical outcomes. The results of this systematic review could provide evidence to help shape future guidelines for using SDF.

Materials and methods

This report follows the PRISMA statement (Preferred Reporting Items for Systematic Reviews and Meta-analyses) [22]. The protocol was registered in PROSPERO (CRD42023393554).

Assessment of the methodological quality of the included studies

The investigators independently conducted the QUIN tool (risk of bias tool for assessing in vitro studies conducted in dentistry) [23] to assess the quality of the included studies. The evaluation of quality was assessed based on a fixed set of bias domains (clear aims/objectives, sample size calculation, sampling technique, comparison group, methodology, operator details, randomization process, outcome measurement, outcome assessor details, blinding, statistical analysis, and results) (Table 1). The final assessment of QUIN was performed by classifying the individual study into the level of bias risk, denoted as low, moderate, or high. To reach a consensus in any cases of discrepancy, the third reviewer was consulted.

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Table 1. QUIN tool (risk of bias tool for assessing in vitro studies).

https://doi.org/10.1371/journal.pone.0306367.t001

Results

Study selection

A total of 175 studies were identified from the literature search in 4 databases, and 42 duplicate articles were removed. The remaining articles were screened based on the titles and abstracts. Irrelevant studies, narrative or systematic reviews, book chapters, and articles in newspapers and periodicals were also excluded. The full texts of 16 articles were retrieved and examined, and 5 articles were ultimately included in this review. The flow chart of the study is shown in Fig 1.

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Fig 1. Flow diagram for the systematic review.

https://doi.org/10.1371/journal.pone.0306367.g001

Study characteristics

The included studies were all in vitro and conducted between 2021 and 2022 (Tables 2 and 3). Four studies used extracted primary teeth with existing dentine caries, whereas the remaining study used extracted permanent teeth with existing dentine caries. All included studies investigated the effects of light curing on SDF-treated carious teeth, compared to SDF without light curing or SDF with different light-curing protocols [15, 19, 20, 24, 25]. However, there was a wide variation regarding specimen fabrication, the duration of SDF application, and the evaluation period.

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Table 2. Characteristics of the included studies.

https://doi.org/10.1371/journal.pone.0306367.t002

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Table 3. Summary of the included studies.

https://doi.org/10.1371/journal.pone.0306367.t003

Quality assessment of the included studies

The studies included in this review exhibit a range of bias risks, encompassing low to moderate risk, as assessed by the QUIN tool. Among the 5 conducted in vitro studies, only 2 demonstrated a low bias risk, with QUIN scores exceeding 70% [15, 20] whereas the remaining 3 studies exhibited a moderate bias risk, as evidenced by QUIN scores falling within the 50% to 70% range [19, 24, 25]. Lack of clear reporting regarding blinding procedures was a prevalent issue across all studies [15, 19, 20, 24, 25]. The quality assessment of the studies is shown in Table 4.

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Table 4. Assessment of the quality of the studies according to the QUIN tool.

https://doi.org/10.1371/journal.pone.0306367.t004

Discussion

Evidence shows that SDF is effective for caries management in primary and permanent dentitions [7]. In the last few years, there has been growing interest in using SDF with the additional light curing due to its capacity to influence the penetration depth and dentine hardness of carious teeth [26]. Based on the results of the current review, no clinical trial was performed to assess the efficacy of light curing following silver-diamine fluoride treatment in arresting carious lesions in primary and permanent teeth. Despite limited information about light curing on SDF’s success, this systematic review revealed the positive effects of light curing on SDF-treated carious teeth in vitro studies.

The increased hardness of dentine indicates a more robust and more dense tooth structure of the carious lesion, which can prevent further progression [27]. Three studies [19, 20, 25] showed a marked increase in dentine hardness under the carious lesion when SDF was combined with light curing as compared to SDF alone. It can be inferred that the increase in dentine hardness is due to the high polarizing capacity of silver, which facilitates the strong bonding with oxygen, phosphorus, and sulfur groups to form a silver oxide, silver phosphate, and silver sulfide after light exposure [28]. Silver sulfide is a dense, black solid that is insoluble in all solvents and contributes to the black staining observed on surfaces treated with SDF [20]. However, Sayed et al. observed a rapid drop in Vickers hardness values (VHNs) beyond 200 μm in SDF with the light curing group in contrast to the gradual decrease of VHN values for the SDF-only group [25]. This result might be associated with the penetration of SDF due to the shorter time and rapid silver reduction induced by light curing. These studies used the VHN to measure tooth hardness due to its avoidance and easy detection of measurement errors [29]. It is essential to recognize that aspects such as sample preparation techniques, sample dehydration, amount and method of the SDF application, lesion depth, and the hardness test’s location could influence the hardness test measurement in vitro studies [19, 30].

Regarding the penetration of SDF on the tooth surface, Toopchi et al. [20], reported that the penetration depth of SDF into the sound dentine beyond the infected dentine was higher in the SDF-only group than in the SDF with the light curing group. Moreover, the silver ion precipitation in the SDF with light curing was significantly higher than in the SDF at the infected dentine; however, no difference was observed at affected and sound dentine. It is possible that the increased reduction of silver solution caused by light curing did not provide enough time for penetration of the carious lesion, thus resulting in less penetration depth and increased silver precipitation at the infected dentine. In contrast, an in vitro study found no significant difference in silver penetration depth between the groups with SDF only and SDF with light curing on a non-carious primary molar [31] The differences in the results may be attributed to the SDF penetration being affected by the extent of demineralization, sample preparation, sample dehydration, and the teeth used in the studies. Karnowakul et al. [15] observed higher mean mineral density differences (mMDD) in the SDF with light curing group compared to without. They found no significant difference in mMDD between 10 seconds of SDF with light curing and 60 seconds of SDF alone, suggesting similar effects on the remineralization process. This implies that SDF with light curing for a shorter duration could be as effective as the longer recommended application time for caries control. The potential mechanism is that dental light curing increases silver ion precipitation in the diseased dentin layer of carious lesions. This layer absorbs more light than deeper layers, with natural light acting as a catalyst in reducing the silver solution, producing silver ions and nanoparticles. Thus, dental light curing serves as an initiating agent, enabling rapid reduction and shorter SDF penetration times. Nevertheless, due to the limited number of five in-vitro studies (4 out of 5 tested on primary teeth) and a lack of clinical studies, the mechanism of using Light curing to improve the clinical outcomes is not yet fully understood. Therefore, a robust conclusion cannot be drawn.

Concerning the antibacterial properties, a recent in vitro study revealed no significant difference in bacterial activity between SDF-treated groups, regardless of the application time or light curing [24]. This finding was consistent with the previous study, which suggested that SDF with light curing had a similar antimicrobial action to SDF [26]. In addition, the shorter application time with light curing does not impede or diminish the antibacterial activity of the SDF as compared to the recommended approach [15].

All of the studies included in this systematic review were in vitro studies. As they all used the extracted natural carious teeth and attempted to simulate the oral environment, the extrapolation of the results to clinical circumstances is limited due to the complex dynamic nature of an oral environment. The included studies had a wide range of sample preparation, characteristics of the extracted teeth, exposure parameters, and evaluation periods. However, the current review of studies has demonstrated the favorable outcomes of SDF with light curing, such as increased microhardness, higher mineral density, a more significant amount of silver ion precipitation in infected dentine, no inhibition of antibacterial properties, and a potential reduction in chair time for caries management. Despite the favorable findings observed in the included in-vitro studies, there are some concerns regarding their susceptibility to false-positive outcomes, limited external validity, and challenges in generalizing their results to real clinical settings. Therefore, future clinical research is needed to warrant or refute the additional benefit or clinical effectiveness of light curing after an SDF application, compared to SDF without light curing. In addition, laboratory studies are necessary to comprehend the mechanism of SDF with light curing and its effect on both carious tooth surface and pulp tissues.

Conclusion

Drawing from the limited number of laboratory studies, incorporating light curing subsequent to the SDF application yields potential favorable outcomes that include augmented microhardness, elevated mineral density, and heightened silver ion precipitation within infected dentine. Future clinical research is required to confirm or refute the benefit of light curing on SDF-treated carious lesions.

Supporting information