Silver diamine fluoride therapy for dental care

Abstract

Silver diamine fluoride (SDF) was developed in Japan in the 1960s. It is used to control early childhood caries, arrest root caries, prevent fissure caries and secondary caries, desensitise hypersensitive teeth, remineralise hypomineralised teeth, prevent dental erosion, detect carious tissue during excavation and manage infected root canals. SDF is commonly available as a 38% solution containing 255,000 ppm silver and 44,800 ppm fluoride ions. Silver is an antimicrobial and inhibits cariogenic biofilm. Fluoride promotes remineralisation and inhibits the demineralisation of teeth. SDF also inactivates proteolytic peptidases and inhibits dentine collagen degradation. It arrests caries without affecting dental pulp or causing dental fluorosis. Indirect pulp capping with SDF causes no or mild inflammatory pulpal response. However, direct application of SDF to dental pulp causes pulp necrosis. Furthermore, SDF stains carious lesions black. Patients must be well informed before SDF treatment. SDF therapy is simple, painless, non-invasive, inexpensive, and requires a simple armamentarium and minimal support. Both clinicians and patients generally accept it well. In 2021, the World Health Organization included SDF as an essential medicine that is effective and safe for patients. Moreover, it can be used for caries control during the COVID-19 pandemic because it is non-aerosol-generating and has a low risk of cross-infection.

1. Introduction

For many years, dentists in countries such as Argentina, Australia, Brazil, China and Japan have used silver diamine fluoride (SDF) as an anti-caries agent [1]. It is effective in arresting cavitated dentine carious lesions and preventing new caries development [2]. SDF is used to arrest early childhood caries [3], manage root caries [4], prevent pit and fissure caries [5], impede secondary caries [6], remineralise hypomineralised teeth [7], treat infected root canals [8] and desensitise hypersensitive teeth [9]. Some clinicians have suggested using SDF to manage dental erosion [10]. Other clinicians have suggested using SDF as a detecting dye for carious tissues to guide caries excavation [11]. Researchers have applied laser irradiation with SDF therapy to prevent tooth fracture after endodontic treatment [12] and to prevent root caries development by increasing fluoride uptake in dentine [13], [14].

Rosenblatt and co-workers called SDF a silver-fluoride bullet for caries management because SDF is a safe, effective, efficient and equitable caries control agent [15]. They concluded that SDF may meet the criteria of caries treatment of the United States Institute of Medicine and the World Health Organization (WHO) Millennium Development Goals. In 2021, the WHO included SDF as one of the most efficacious, safe and cost‐effective medicines for meeting the most important needs in a health system for adults and children [16]. Considering the growing evidence of successful SDF therapy, it has become a popular clinical treatment in recent years. A survey in the United States found that paediatric dentistry residency programmes had universally adopted SDF for arresting caries in 2020, but only a quarter (26%) of them reported using SDF in 2015 [17]. SDF has also attracted researchers’ attention since 2016. A recent bibliometric analysis found that global research interest regarding SDF increased exponentially in the five years from 2016 to 2021 [18]. Furthermore, there has been a steady increase in scientific research on SDF, which has been translated into clinical practice to improve the quality of dental care. The objective of this review paper is to provide an overview of SDF use in dentistry.

2. History of SDF use in different countries

Professor Misuho Nishino at Osaka University formulated a 38% SDF solution during her PhD studies in the 1960s [19]. She and her supervisor, Professor Reichi Yamaga, reported the clinical applications of SDF in 1972 [6]. The Central Pharmaceutical Council of Japan’s Ministry of Health and Welfare accepted SDF as a therapeutic agent for dental treatment [3]. It is manufactured in Japan as a 38% SDF solution and a 3.8% SDF solution, which are marketed as Saforide and Saforide RC, respectively, for dental treatment.

In China, Professor YJ Li at Beijing Capital Medical University developed a low-cost 38% SDF solution for arresting caries. She reported the effectiveness of SDF in arresting caries in 1984 [20]. Dentists in some hospitals also used SDF to treat dentine hypersensitivity [9]. However, the use of SDF became unpopular in China, probably because dentists could not charge for caries-arresting treatment with SDF. Few publications regarding SDF can be found in the literature before 2000 [21], [22]. In 2002, a research team searched for alternative treatments for caries management. Based on a PhD study, they found that SDF could arrest caries without excavating the soft, active carious tissue [23]. The research team subsequently conducted several seminal laboratory studies [24], [25], [26], [27], [28], [29] and clinical trials [30], [31], [32], [33], [34], [35], which have contributed to an understanding of caries arrest using SDF. In 2019, Professor C H Chu at the University of Hong Kong implemented a dental outreach service programme using SDF to arrest dental caries to preschool children. All the 180,000 kindergarten children were eligible to receive SDF therapy with parental consent in their kindergartens.

In Australia, Dr Graham Graig at the University of Sydney used 40% neutral silver fluoride solution to treat caries. He reported the silver fluoride solution could slow down caries in primary teeth in 1980s [36]. The Western Australia School Dental Service used silver fluoride solution to arrest caries of school children. Because the silver fluoride-stained caries black, Dr Geoffrey Knight in the 2000s at the University of Adelaide used potassium iodide to remove the black staining [37]. A manufacturer subsequently developed SDF with potassium iodide (Riva Star). Although Riva Star was approved as a tooth desensitising agent in Australia, dentists use it to treat dental caries.

In the United States, Professor Peter Milgrom at University of Washington studied SDF for management of dental caries and dentine hypersensitivity [38], [39]. In 2014, the United States Food and Drug Administration cleared the use of SDF for dentine desensitisation in adults over 21 years of age. Since 2015, SDF has been manufactured in the United States as a 38% solution. It is commercially available as Advantage Arrest as an anti-hypersensitivity agent for dental treatment, and many dentists use SDF for caries treatment. In 2016, the United States Food and Drug Administration granted the manufacturer breakthrough therapy status as part of the process of seeking approval for SDF as a drug formally indicated for arresting dental caries. In 2017, Health Canada approved the use of a product of SDF (Advantage Arrest) for caries treatment. An implementation research team partnered with the First Nations Health and Social Secretariat of Manitoba and the Manitoba Metis Federation to use SDF for management of early childhood caries in indigenous communities in Manitoba, Canada [40]. In addition, several other Indian Health Service and tribal programs in Alaska now use SDF for caries management with promising results [41].

In Cambodia, Professor Durward at the University of Puthisastra and Dr Turton conducted the Healthy Kids Cambodia project in 2016 [42]. The aim of the project is to provide a sustainable and integrated approach to primary health care for school children in Cambodia. The project incorporated the use of SDF as an essential treatment intervention to achieve universal oral health care [43]. The participated children received 30% SDF (Cariestop) to arrest dental caries on their primary teeth after screening.

In Thailand, the Dental Association of Thailand launched a guideline for fluoride usage including SDF. In 2018, the Thailand Food & Drug Administration approved a commercial product of 38% SDF (Topamine) for treatment of dentine hypersensitivity. The Ministry of Public Health Thailand subsequently approved SDF for treatment of coronal caries in primary teeth and root caries in permanent teeth. Dentists can use SDF as a professional fluoride therapy which is covered by the Universal Health Coverage in Thailand.

In 2020, the American Dental Association supported the use of SDF for caries management [44]. A national billing code (D1354) exists for dentists in the United States to use to charge for SDF therapy. Another billing code (D1355) was approved to SDF for caries prevention. Moreover, government and private insurance programmes cover SDF therapy for dental care. The American Medical Association is considering a Current Procedural Terminology (CPT) code for SDF therapy. The CPT is a standardised code set to report medical services and procedures to physicians, insurance companies and accreditation organisations.

In 2021, the WHO included SDF in the WHO Model List of Essential Medicines for both adults and children [16]. The WHO considers that SDF is a medicine to which everyone should always have access and that all governments should ensure SDF is available and affordable to their populations. Unfortunately, SDF is not yet available in all countries. Although government organisations and dental associations have developed guidelines for clinical use of SDF [1], some countries have no guidelines for SDF use. Table 1 shows guidelines and current usage of SDF in some countries. Because SDF is not available in some countries, dentists use silver nitrate solution first, then apply sodium fluoride varnish to arrest dental caries [45].

Table 1.

Countries and guidelines for silver diamine fluoride (SDF) use (adapted from Gao et al., 2021 [1]).

Countries	Guidelines for SDF use (Reference number)
Africa
Egypt	No
Ghana	No
Kenya	No
Nigeria	No
South Africa	No
Asia
India	No
Japan	Clinical practice guideline for caries treatment [46]
Mongolia	Standard guidance and recommendation for SDF [45]
Thailand	Guideline on caries diagnosis and management [47]
Europe
Finland	National guidelines for caries management [48]
Switzerland	Clinical guidance and recommendation for caries management [49], [50], [51], [52]
United Kingdom	•Standard operating procedure (England) [53] •New SDF resources by the British Society of Paediatric Dentistry [54]
North America
Canada	New guidance in treating caries by the Canadian Dental Association [55]
United States	•Guidelines for SDF use by the American Academy of Pediatric Dentistry[56] •Evidence-based clinical practice guideline on nonrestorative treatments for carious lesions by the American Dental Association [57]
South America
Argentina	No (Individual organisations have their own recommendations)
Brazil	•National protocol in primary health care for the Brazilian public health system [58] •Guidelines for clinical procedures in paediatric dentistry [59]
Oceania
Australia	Australian fluoride guidelines [60]

Several manufacturers have developed and marketed SDF solutions with various concentrations for dental use (Table 2). SDF is generally used as a 38% solution for treating caries and dentine hypersensitivity. Studies have found that SDF is significantly less effective in arresting caries at a low concentration (10–12%) than that at a high concentration (38%) [34], [61]. A manufacturer has developed 3.8% SDF solution (Saforide RC, Toyo Seiyaku Kasei Co. Ltd., Osaka, Japan) as a disinfection agent for root canal therapy.

Table 2.

Concentrations of some commercially available SDF solutions for dental use.

SDF%	Product	Manufacturer	Country
38%	Advantage Arrest	Elevate Oral Care	US
38%	Bioride	DENTSPLY Industrial & Commercial Ltd.	Brazil
38%	e-SDF	Kids-e-Dental	India
38%	Dengen Caries Arrest	Dengen Dental	India
38%	Fagamin	Tedequim SRL	Argentina
38%	Fluoroplat	NAF Laboratory	Argentina
38%	Riva Star	SDI Dental Ltd.	Australia
38%	Saforide	Toyo Seiyaku Kasei Co. Ltd.	Japan
38%	Topamine	Dentalife Australia Pty. Ltd.	Thailand
30%	Ancárie	Maquira, Maringá	Brazil
30%	Cariestop	Biodinamica	Brazil
12%	Ancárie	Maquira, Maringá	Brazil
12%	Cariestop	Biodinamica	Brazil
10%	Cariestatic	Inodon Dentistry	Brazil
3.8%	Saforide RCa	Toyo Seiyaku Kasei Co. Ltd.	Japan

^aFor root canal therapy

3. Ingredients and concentrations of SDF solution

SDF is a colourless alkaline solution with a pH value between 9 and 10. It can be tinted blue to facilitate SDF application. Although the manufacturers have not disclosed details about the ingredients [62], their SDF products basically contain silver, fluoride and ammonia. Silver fluoride is unstable. Ammonia is added to form diammine silver fluoride [Ag(NH₃)₂⁺]F. Diammine silver fluoride has a linear structure which is more stable than that of silver fluoride. Like most silver compounds, SDF is unstable under light irradiation and decomposes to silver. Hence, SDF must be stored in a light-proof bottle. It should be used as soon as possible once it is dispensed from the bottle in clinical practice.

Although manufacturers produce SDF according to their own formulation, researchers mostly assume that a 38% SDF solution contains 253,870 ppm silver and 44,800 ppm fluoride ions. In other words, a 38% SDF solution is composed of 25% silver ions and 5% fluoride ions dissolved in an 8% ammonia solution. However, studies have found significant variations in silver and fluoride ion concentration in different SDF products [63], [64], [65]. This might be because a 38% SDF solution could be any combination of silver, fluoride and ammonia that totals 38%. Because the concentration of silver, fluoride and ammonia falls within an average range, SDF may not be exactly 38% [66]. In addition, the studies measured the SDF solutions’ fluoride concentration via an ion-selective electrode method. This method is non-contaminating and has a short response time. Nevertheless, its precision is not high and the measurement for SDF in the studies could be less robust because of the small samples’ dilution.

4. Safety of SDF

Dentists have been using SDF since the 1960s with success for caries arrest. Apart from staining of the carious lesion, literature reported no other significant complications associated with SDF therapy. A clinical trial on 375 young children found that no severe reaction after SDF application. Another clinical study on 126 adults found that no severe erythema after SDF application [39]. SDF can cause transient gingival irritation requiring no treatment [67]. A systematic review concluded that SDF causes mild, reversible pulpal inflammation and is generally biocompatible [68]. A clinical study on six healthy adults found that the fluoride concentrations in serum after SDF application posed little toxicity risk [69]. Another study on 55 children found SDF is safe, and serum concentrations of fluoride pose little risk of toxicity [70]. A study showed that the risk of fluoride toxicity in a young child is low even SDF is applied to all the 20 primary teeth [71].

5. Properties of SDF solution

When silver combines with fluoride in an ammonia solution, silver and fluoride ions are released. Silver is antibacterial. Bacterial resistance against silver is hard to develop because silver kills bacteria through multiple pathways [72]. Silver interferes with bacterial metabolism. It is an oxidising agent that interacts with the sulfhydryl group of proteins and the deoxyribonucleic acid (DNA) of bacteria. The interaction impedes cellular respiratory processes, alters cell wall synthesis, unwinds DNA, and inhibits cell division [73]. Silver also inactivates the bacterial enzyme glycosyltransferase, thus inhibiting biofilm formation. Glycosyltransferases synthesise glucan, which is responsible for bacterial adhesion and the thickening of biofilms on tooth surfaces [74]. Silver also inhibits the proteolytic activities of collagenases to prevent degradation of dentine collagen [62]. In addition, silver reduces bacterial adhesion by incorporating silver into hydroxyapatite to form silver-containing hydroxyapatite in enamel and dentine [75], [76].

Fluoride is well known as a remineralisation agent. Fluoride can react with hydroxyapatite to form fluoride-substituted hydroxyapatite via ion exchange of fluoride ions for hydroxyl ions and/or crystal growth of fluorapatite from saliva [62]. Fluoride-substituted hydroxyapatite is more acid-resistant than hydroxyapatite. Hence, enamel or dentine with fluoride-substituted hydroxyapatite is resilient to acid attack. Furthermore, fluoride can precipitate as calcium fluoride. Calcium fluoride is adsorbed and loosely bound onto the tooth surface. Although researchers have various views on this loosely bound calcium fluoride in the oral cavity, many accept that the calcium fluoride can act as a depot and release fluoride ions under an acidic environment to form fluorohydroxyapatite steadily [77]. Fluoride at a high concentration has antimicrobial effects on cariogenic bacteria [78]. Fluoride ions can bind to bacterial enzymes to inhibit the carbohydrate metabolism of acidogenic bacteria as well as their sugar uptake [79], [80].

Researchers generally accept that silver and fluoride have a synergistic caries-arresting effect [20], [62]. Reviews have concluded that SDF inhibits biofilm formation, promotes remineralisation, counteracts demineralisation, prevents collagen degradation and occludes dentinal tubules [62], [81]. These desirable properties of SDF make it an effective agent for management of dental caries and dentine hypersensitivity. Most SDF solutions are alkaline with a pH value of 9–10. The strong alkalinity contributes to the inhibition of the proteolytic activities of collagenases, which can break down dentine collagen. The alkalinity of Riva Star is very high with a pH value of about 13, which can burn the gingiva. To reduce the risk of gingival irritation, the manufacturer produced an ammonia-free silver fluoride solution (Riva Star Aqua). Because Riva Star Aqua is an aqueous silver fluoride solution without the ammonia base, it tastes and smells better than Riva Star.

5.1. SDF inhibits bacterial growth and biofilm formation

SDF inhibits the growth of cariogenic bacteria [79], [81]. The minimum inhibitory concentration and minimum bactericidal concentration of SDF for S. mutans are lower than that of silver ammonium nitrate and sodium fluoride, showing that SDF is more effective compared to silver ammonium nitrate and sodium fluoride in inhibiting bacterial growth. SDF possesses strong antimicrobial action against S. mutans, A. naeslundii and L. acidophilus [24], [82]. S. mutans is an important pathogen associated with the initiation and progression of carious lesions. SDF can inhibit the adherence and growth of S. mutans on the surface of carious lesions [82]. L. acidophilus is often found abundantly in dentine carious lesions [81]. A. naeslundii can invade rapidly through dentinal tubules and is attributed to the development of root caries. Studies have shown that SDF inhibited the growth of multispecies cariogenic biofilms containing S. mutans, A. naeslundii, and L. acidophilus on the tooth surface [79], as well as E. faecalis biofilm on root canals [81]. An in vitro study found silver particles, together with dead bacteria on the dentine surface after SDF application [82]. Although SDF is an antibacterial, two ex vivo studies found no significant change in the overall microbiome in the arrested carious lesions treated with SDF [38], [83]. However, these two studies had a small sample size, and more well-designed clinical studies are required to validate the results.

5.2. SDF promotes remineralisation and counteracts demineralisation

Arrested carious lesions treated with SDF generally have a black and hard surface. This clinical observation is corroborated by an ex vivo study that reported an increase in microhardness of the dentine surface layer after SDF application [84]. Laboratory studies found the surface layer of dentine carious lesions was rich in calcium and phosphorus after SDF treatment [29], [84]. Saliva is supersaturated with calcium and phosphate. In the presence of saliva, SDF fosters remineralisation of teeth in the oral cavity when the pH value is higher than 5.5 [78]. SDF also inhibits calcium dissolution from hydroxyapatite and prevents the demineralisation of enamel and dentine [85]. A study found that enamel carious lesions, after SDF application, had significantly less mineral loss compared to those without SDF treatment [86]. The plaque on the tooth surfaces absorbs fluoride after SDF application. When bacteria produce acids, the fluoride in the plaque fluid, along with the acids produced, penetrates subsurface of enamel. The fluoride is adsorbed to the crystal surface and protects the crystal from dissolution [87]. In addition, calcium fluoride, silver phosphate and silver protein can be formed and precipitate on the dentine surface after SDF application [88]. A study showed these precipitates developed dense granular structures of spherical grains on the intertubular area of dentine and occluded orifices of dentinal tubules [89]. The precipitates containing high silver and phosphorus content decreased calcium and phosphorous loss from the dentine carious lesions. Although silver phosphate forms in the precipitates, it is relatively unstable, and silver chloride will replace it [24], [90]. This is because the solubility product of silver chloride is lower than that of silver phosphate [62].

5.3. SDF inactivates proteolytic activity and prevents collagen degradation

The pathogenesis of enamel caries and dentine caries are not the same. Enamel predominantly consists of the mineral hydroxyapatite (95%) with trace amounts (>1%) of organic matter by weight percentage. Dentine contains approximately 70% mineral by weight, mainly hydroxyapatite; 20% organic matrix, mainly type I collagen; and 10% fluid. The type I collagen in dentine forms a structural backbone that holds the hydroxyapatite together [81]. In an acidic environment, such as an active carious lesion, or when cariogenic bacteria produce lactic acid, collagenases are activated which destroy dentine collagen. SDF has an inhibitory effect on collagenases, such as matrix metalloproteinases (MMPs) and cysteine cathepsins [25], [91]. MMPs are endopeptidases found in the dentine matrix and in saliva. MMPs, in particular MMP-2 (gelatinase A), MMP-8 (neutrophil collagenases) and MMP-9 (gelatinase B), play a crucial role in collagen breakdown in carious lesions. Cysteine cathepsins are proteolytic enzymes found in dentine carious lesions and dental pulp. Cathepsin B can break down collagens, and cathepsin K can catabolise collagen. Activation of MMPs and cysteine cathepsins contributes to collagen breakdown in dentine carious lesions. Inactivation of collagenases and prevention of collagen degradation contributes to the arrest of carious lesions using SDF [91], [92], [93].

5.4. SDF occludes dentinal tubules and promotes tertiary dentine formation

The fluorohydroxyapatite produced after SDF application promotes remineralisation, which can block or decrease tubule diameter, resulting in relief of dentine hypersensitivity [81]. Silver also precipitates as silver salts on the dentine surface and within the dentinal tubules after SDF application [94], [95]. Studies have found that the silver deposits can penetrate deep into dentinal tubules [96], [97], [98]. The primary mineral precipitates that form after SDF application are calcium phosphates and silver salts [62]. The calcium phosphates can be needle-shaped precipitates that contain calcium, fluoride and phosphorus. The silver salts can be crystals containing silver and chloride [99]. A study found that the precipitated particles became angular and larger, suggesting the presence of secondary crystallisation [98]. Although SDF can penetrate very deep into dentine through patent dentinal tubules, a systematic review concluded that SDF caused no or mild inflammatory response of dental pulp, even though the remaining dentine was thin (0.25–0.50 mm) [100]. A study observed increased cellular activity of the odontoblasts in the dental pulp with the formation of tertiary dentine after SDF application [101]. Thus far, little is known regarding the presence of bacteria within the dentine–pulp complex when SDF is applied to deep carious lesions. A case report found no viable microorganisms within dentine or inside the tooth pulp after SDF application [97].

6. Clinical applications of SDF

Dentists use SDF to desensitise hypersensitive teeth, manage dental caries, remineralise hypomineralised molars, disinfect root canals, manage dental erosion and treat gross gingivitis. They favour SDF therapy because of its ample advantages. First, SDF is effective in arresting caries. Hence, it can be used as non-restorative caries management for controlling pain and infection. Second, is inexpensive. Numerous manufacturers produce SDF at an affordable cost. Third, SDF therapy is straightforward. SDF therapy’s simplicity allows dentists, physicians and other qualified health personnel such as hygienists to apply SDF with simple training. Fourth, SDF therapy is painless. Patients generally accept the painless application well, which helps to alleviate patients’ fear and anxiety and develop cooperation during treatment. It is particularly useful for treating young children, older adults and people with special needs. Fifth, SDF therapy is non-invasive and non-aerosol-generating. In particular, it can be used to control caries during the COVID-19 pandemic because of its low risk of cross-infection. Finally, SDF requires minimal support and does not require sophisticated or expensive equipment. The simple armamentarium allows clinicians to deliver SDF therapy as an outreach service to people who live in remote areas.

The main disadvantage of SDF therapy is permanent black staining on carious lesions. The staining is primarily to active dental caries. Stains to sound enamel and root dentine are minor and can be polished away. Patients must be well informed before treatment to avoid patient dissatisfaction. SDF also stains the skin, working table, dental instruments and clothes if it is handled carelessly. The stain SDF causes on clothes and dental instruments can be permanent. The stain on the skin often lasts for a few weeks. Although it does not cause any pain, stains on patients’ lips, chin or face or the workers’ hands are unaesthetic, affecting their social life. Handling with care and using a tinted SDF solution helps prevent this mishap. A scoping review concluded that the staining of arrested carious lesions in children did not influence parental satisfaction [102]. A manufacturer suggests a two-step process using potassium iodide to reduce staining under restorations. The potassium iodide treatment is intended for use when a tooth is being restored, often with a sandwich restoration. When the caries lesion is left exposed to light, there is staining even when potassium iodide is used A clinical trial found the application of potassium iodide in the long term does not reduce the blackening of arrested caries lesions caused by SDF [103]. Dentists can use glass ionomer cement or composite resin restoration to restore the SDF-treated lesion at another appointment. In this case, the SDF serves to obviate the need for local anaesthetic.

SDF also has an unpleasant metallic taste. Some clinicians apply fluoride varnish on the top of the SDF to mask the unpleasant taste of SDF [104], [105]. The varnish can also be a protective layer to prolong contact of the SDF with the tooth surface. Proper isolation of the tooth can avoid SDF contacting the tongue and enhance the patient’s acceptance of SDF therapy. Furthermore, SDF may induce transient gingival and mucosal irritation. In general, the irritation will spontaneously heal and require no treatment. Direct application of SDF to dental pulp causes pulp necrosis [100]. However, indirect SDF application generally does not cause a significant inflammatory response of dental pulp [101]. A 30-month clinical trial on young children found SDF therapy did not cause acute systemic side effects [106]. In addition, toothache or gum pain, gum swelling and gum bleaching after SDF application were uncommon. In general, studies in Hong Kong and the United States reported that parents generally accepted SDF therapy for their children [107], [108], [109].

6.1. Treating dentine hypersensitivity

Dentine hypersensitivity arises from exposed dentine surfaces in response to stimuli such as cold and touch. Dentine hypersensitivity can result in severe, persistent pain and discomfort [110]. Laboratory studies have found evidence of occlusion of dentinal tubules of dentine after SDF application [94], [98], [111]. Clinical studies showed topical application of SDF relieved pain in adults with hypersensitive permanent teeth within one week [39], [112], [113]. A 38% SDF gel was developed to offer clinicians better control of the application area on the hypersensitive tooth [98]. Some dentists apply SDF on a vital tooth in preparation for extra-coronal restoration to reduce post-operative sensitivity. Glass ionomer and resin-modified glass ionomer can be used as luting cements because SDF does not affect their bond strength [26], [114]. A study reported that SDF adversely affected the bond strength between dentine and some resin adhesives [115].

6.2. Managing dental caries

SDF is an archetypal example of non-restorative caries treatment that steers sustainable caries management through controlling bacterial infection and remineralising teeth. Dentists use SDF to arrest early childhood caries. A systematic review reported that the overall rate of caries arrest was 81% in primary teeth [2]. Some researchers proposed using SDF for caries prevention [116]. However, an umbrella review concluded there was insufficient evidence to draw conclusions on SDF for prevention in primary teeth and prevention and arrest in permanent teeth in children [117]. More well-designed clinical trials are needed to evaluate SDF’s effectiveness in preventing caries development. SDF is also used to manage root caries. Root caries are very challenging to restore due to the proximity to the pulp and the location of the carious lesion [103]. Current evidence shows SDF therapy is an effective fluoride agent for arresting root caries [118], [119], [120], [121]. Annual SDF application is a simple, inexpensive and effective way of preventing the initiation and progression of root caries in older adults [118]. A systematic review found the preventive fraction of SDF for preventing root caries ranged from 50% to 68% [119]. Two reviews found that annual application of 38% SDF was the most effective treatment among the different treatments for preventing root caries [120], [121].

6.3. Remineralising hypomineralised molars

Hypomineralisation of teeth is a developmental defect that usually affects permanent incisors and first molars [122]. Well-demarcated opacities, which can be chalky white to dark brown in colour, characterise clinically hypomineralised teeth [123]. Histologically, the opacities are porous and located in the inner part of the enamel. Children with hypomineralised teeth often suffer from severe hypersensitivity due to rapid tooth wear and exposure of dentine. They also have a high risk of caries because of defective enamel in the hypomineralised teeth. SDF is a simple, painless and non-invasive therapy for managing hypomineralised teeth. SDF application effectively desensitises a hypomineralised tooth and quickly arrests carious lesions on it. A clinical study found biannual SDF application prevented caries development and reduced dentine hypersensitivity in hypomineralised molars [123]. The study concluded that SDF application continued through repeated applications on hypomineralised teeth provides profound and long-lasting relief of dentine hypersensitivity.

6.4. Disinfecting a root canal in endodontic therapy

Chronic periapical periodontitis is a common endodontic infection that causes failure of endodontic treatment. SDF is available at a low concentration of 3.8% for managing endodontic infection. The infection is often associated with E. faecalis [124]. Studies found irrigation with a 3.8% SDF solution in endodontic treatment was effective in removing E. faecalis biofilms in root canals [125], [126]. SDF disinfects and prevents reinfection of the root canals. SDF can absorb and kill dentinophilic bacteria in the circumpulpal dentine [126], [127]. The silver salts of SDF precipitate and obliterate the dentinal tubules and prevent bacteria from reinfecting the disinfected dentinal tubules. Researchers suggested the silver salts may provide a potential means of substantivity that permits the gradual release of silver ions over time. Furthermore, researchers reported that SDF decreased the permeability, increased the surface hardness and strengthened the fracture resistance of the root canal [6], [12], [128]. However, clinical studies verifying the results of the laboratory studies are lacking.

6.5. Managing dental erosion

Dental erosion is an acid dissolution of tooth structure that does not involve bacteria [129]. The loss of tooth structure with open dentinal tubules often initiates dentine hypersensitivity, which can result in severe, persistent pain and discomfort. SDF is an effective anti-erosive agent with remineralising properties [84]. It reacts with the teeth’s minerals to form metallic silver and silver chloride on tooth surfaces [24], [25]. A study found silver-containing hydroxyapatite on enamel after SDF application [27]. These silver composites’ formation plausibly enables SDF to prevent dental erosion. A laboratory study found SDF was effective in reducing dental erosion in enamel and dentine [10]. SDF can be used for preventing dental erosion once its efficacy is confirmed in clinical studies.

6.6. Treating gross gingivitis

A laboratory study reported that SDF demonstrated significant antibacterial properties against putative periodontal pathogens causing severe periodontitis [130]. The authors suggested that SDF could be a potential new therapeutic agent for management of periodontal infections because SDF suppressed periodontal pathogens in subgingival biofilms. A clinical study reported that SDF can be used to treat hyperplastic gingivitis [131]. The authors found SDF could effectively reduce marginal gingival and papillary inflammation. Another one-year clinical trial found that SDF improved the gingival health of older adults with gingivitis with no adverse effects [132].

7. SDF therapy in outreach service

Conventional dental treatment for dental caries is not accessible, available or affordable in many communities. Using SDF in outreach service can be a pragmatic strategy for caries management [133]. SDF therapy is simple, non-invasive and inexpensive and does not require a sophisticated armamentarium. It is particularly useful for dental outreach services to prevent and arrest caries. Hong Kong and Mongolia are offering region-wide large-scale dental outreach services using SDF to control early childhood caries in preschool-age children. Apart from young children, SDF therapy is a promising strategy for managing dental caries in older adults and those who have special needs when physical limitations do not allow conventional dental treatment. In addition, SDF is non-aerosol-generating and has low risk of cross-infection. It can be used for caries control during the COVID-19 pandemic.

8. Conclusion

In conclusion, SDF is a caries-arresting fluoride agent with antibacterial and mineralising properties. It can inhibit biofilm formation, promote remineralisation, counteract demineralisation, prevent collagen degradation and occlude dentinal tubules. It can be used to arrest early childhood caries, manage root caries, prevent fissure caries, impede secondary caries, desensitise hypersensitive teeth and manage infected root canals. SDF therapy to arrest caries is simple, painless, non-invasive, non-aerosol-generating and inexpensive. Both clinicians and patients generally accept SDF therapy well.

Funding

This research was funded by General Research Fund of the Hong Kong Research Grant Council, grant number 17100019.

Conflict of interest

The authors declare that they have no conflict of interest.