Minimally Invasive Techniques Used for Caries Management in Dentistry. A Review

A. Silver Nitrate (Sn) + 5% Fluoride Varnish (Naf) For Caries Management

The use of silver nitrate (SN), whose chemical formula is AgNO3, was reported to be the first silver compound used for arresting caries in the 1840s [21]. In 1917, Howe proposed a simple procedure to generate metallic silver, which was later known as Howe’s ammoniacal silver nitrate solution (AgNH₃NO₃) [23]. Silver nitrate was then adopted for caries management [23]. It was reported that silver nitrate could arrest dental caries on both permanent and primary teeth [23]. Metallic silver within Howe’s solution was believed to penetrate into affected dentin and have an antibacterial effect. However, Howe’s solution was only used up to the 1950s. Doubts of its clinical efficacy and possible adverse effects were mentioned by several studies concluding that it could penetrate vital and non-vital dentine; it was also said to have a self-limiting and localized effect on the pulp [23]. Silver nitrate can be toxic if ingested; however, silver nitrate in very low concentrations is used in dentistry. For example, the commonly used 25% SN contains 0.25 grams of silver nitrate in a 1 milliliter solution, which is equivalent to 20 drops [21]. Since each patient often needs only one to two drops during the treatment, the level of silver nitrate exposed to patient is extremely low. Countries, such as Japan, China, and Australia, have used silver nitrate in dentistry for decades. As silver nitrate can penetrate into vital and non-vital dentin, it is limited to teeth that have no pulpal infection and abscesses to prevent inflammatory response. Today, silver is again favored as an antimicrobial agent due to its low toxicity and bacterial resistance [22].

Caries can progress and demineralize enamel and dentine that usually leads to pulpal infection. Yet, its progression can be minimized through remineralization. Sodium fluoride promotes remineralization and silver nitrate has a profound antimicrobial effect [21]. A laboratory study investigated the use of silver nitrate solution on artificial caries lesions in a microbial biofilm model [27]. The results showed that caries progression (lesion depth) was significantly lower with silver nitrate application when compared with no treatment. Another study showed that silver nitrate inhibited the growth of all tested oral bacteria [28]. Silver nitrate was thus suggested to have strong antibacterial action.

Other studies have suggested that the combined application of silver nitrate solution followed by sodium fluoride varnish can be used to arrest dental caries [21]. Duffin proposed the use of ions from 25% SN solution followed by 5% NaF varnish application as a non-invasive procedure to treat caries in children [29]. The results showed that almost all caries lesions were arrested after treatment. Radiographic examination showed the formation of secondary dentin. Subsequent restorations could be done without the need for local anesthesia, which also suggested that secondary dentine was formed. The clinical follow up showed that almost all (98%) of carious lesions remained arrested for up to four years after treatment [29]. An in-vitro study also confirmed that the caries-arresting effect of a 25% SN solution followed by 5% NaF varnish application enhanced the remineralization process of artificial caries-affected dentine and inhibited the degradation of dentin’s collagen [30]. A study using scanning electron microscopy showed that the teeth that received 25% SN + 5% NaF had no dentine collagen exposed, with smaller mean lesion depth, and lower hydroxyproline released from dentine when compared to the control non-treated teeth [30]. The application of silver nitrated followed by fluoride vanish was shown to be an effective approach in caries management. Moreover, the treatment protocol is simple, non-invasive, and low-cost. It was reported that the SN only costed about $32 for one 8-mL bottle (~$4.00/mL) [31]. This makes silver nitrate a potential strategy for treating dental caries in young children, elderly populations, and people with special needs in the low-income communities [21].

To apply the combined technique of silver nitrate and fluoride varnish, assure that the patient is not allergic to silver. It is recommended to take radiographs before the treatment to make sure there is not a compromised pulp and the tooth is not abscessed. A microbrush can be used to apply tiny amounts of the silver nitrate into the affected tooth structure, followed by the application of sodium fluoride varnish as a separate treatment at the same appointment. This technique promotes remineralization of damaged tooth surfaces and treats unaffected tooth surfaces. The teeth can then be restored for cosmetic reasons, as needed.

B. Silver Diamine Fluoride (Sdf) For Caries Management

Silver products such as silver nitrate (SN) and silver diamine fluoride (SDF) have been used in Japan for over 40 years to arrest caries in primary and permanent teeth. In the past decade, many other countries, such as Australia and China, have been using SDF with similar success [32, 33]. SDF was recently introduced in the United States, but it wasn’t until 2014 that it was cleared for use in the U.S. by the Food and Drug Administration [33]. SDF is a practical and affordable treatment for arresting the progression of carious lesions in primary and permanent teeth [34]. According to a presentation by Association of State and Territorial Dental Directors, the average cost for one 8 mL bottle of SDF is $149 or ~$18.63/mL [35].

SDF has been used internationally for treating dentinal hypersensitivity and caries lesions. It is a cost effective, minimally invasive, and is a clinically simple treatment option for most of patients. Due to its advantages (non-invasive and easily performed), it can be a promising strategy to manage dental caries in very young children and those with special needs. This product is colorless and odorless. Although it looks like water, it can stain skin, clothing, countertops, flooring, and instruments. SDF is commonly used at a high concentration [38% (44,800 ppm fluoride)] and is effective in arresting caries among children [36, 37, 38]. This concentration is still relatively small since a patient only needs one or two drops for the treatment. SDF is also available in the U.S. market as it was approved by the FDA in 2014 [33]. A meta-analysis review showed an overall percentage of active caries on primary teeth became arrested was: 86% after 6 months of receiving 38% SDF treatment, 81% after 12 months, 78% after 18 months, 65% after 24 months, and 71% after 30 months or beyond [37]. The patients who may benefit from SDF include those with high-caries risk, behavioral or medical management challenges, multiple teeth with cavitated lesions, and those with difficulty accessing dental care [39].

In its 38% concentration, SDF is composed of 25% silver ions and 5% fluoride ions dissolved in 8% amine (2NH₃) that is similar in structure to silver nitrate (SN) followed by fluoride varnish (NaF). SDF’s biochemical interaction with tooth structure and cariogenic bacteria is also similar to SN. Both help strengthen the crystalline structure of the tooth by forming fluorapatite and use silver ion to decrease bacterial deoxyribonucleic acid (DNA) and proteins [40]. It is hypothesized that the caries-arresting ability of SDF is a combined effect of inhibiting cariogenic biofilm, preserving collagen from degradation and increasing the hardness of dentine [41]. In-vitro studies have demonstrated that SDF reduces demineralization and has antimicrobial action on cariogenic bacteria [42]. Silver ions in SDF could inhibit the growth of cariogenic oral bacteria by denaturalizing enzymes that would breakdown collagenous dentin [28]. Streptococcus mutant, a primary pathogen in dental caries, was shown to be less able to form a biofilm on teeth treated with silver fluoride [43]. The fluoride of SDF promotes deposition of fluorapatite, which is more resistant to acidic degradation than normal tooth structure [33].

It is theorized that SDF reacts with hydroxyapatite in an alkaline environment to form calcium fluoride (CaF₂) and silver phosphate as major reaction products. CaF₂ provides the formation of fluorapatite, which is less soluble than hydroxyapatite in an acidic environment [33]. Silver and fluoride ions can penetrate approximately 25 microns into enamel and 50 to 200 microns into dentine. In fact, SDF has arrested lesions of 150 microns depth according to some studies [40].

Systematic reviews have reported that silver compounds are useful in caries management to prevent and arrest caries lesions in primary and permanent dentition. Many studies have highlighted the black dental stains that appear after SDF application as one of its disadvantages [23, 44]. More recent studies [45] have demonstrated its effectiveness. A study on 375 preschoolers demonstrated that annual application of SDF was more effective at arresting caries than application of 5% sodium fluoride every three months [46]; another study of 976 Nepalese schoolchildren demonstrated that a single application of SDF was effective in arresting caries after six, 12 and 24 months [47].

The technique used for SDF application can be modified to meet the clinical needs of individual patients. A suggested SDF protocol for patients with behavioral challenges is to dry, apply and protect. Clinicians are advised to check in one to three weeks post-application to see if caries have been arrested. If the lesion is not arrested, SDF should be reapplied. If arrested, the agent should be reapplied every three to six months [45].

Application Technique for Silver Nitrate & Silver Diamine Fluoride [42]

• Isolate areas to be treated with cotton rolls. Air-drying prior to application is thought to improve effectiveness.

• Remove debris from cavitation. Carious dentin removal is not necessary. When removal is done, it may reduce proportion of arrested caries lesions that become black, so it may be considered for aesthetic purposes.

• Dry lesion with compressed air.

• Dip a brush into SDF or SN solution and apply directly only on tooth surface.

• Allow 1-3 minutes for the silver diamine fluoride to react with the caries lesion.

• Apply gentle flow of compressed air until SDF or SN is dry.

• Remove any excess material with the same cotton used to isolate. This routine minimizes systemic absorption.

C. Disadvantages and Safety Issues on Using Silver Compounds in Dentistry

Studies confirmed that silver compounds were effective to different approaches already mentioned, specifically their capability to arrest dental caries [21, 28, 38, 42, 43]. However, excess of silver is reported to accumulate in skin, liver, kidneys, spleen, corneas, gingiva, mucous membranes, and nails. In exposed parts of body subjected to light, prolonged exposure to large amounts of silver may lead to irreversible pigmentation of the skin or the eyes. However, very little data reported the possible toxic effects of using silver compounds. Traditionally, it has been thought that silver ions have low cell toxicity. There are few known reports of silver allergy [23, 48].

The most evident disadvantage of using silver compounds such as silver nitrate and silver diamine fluoride in dentistry is that the caries lesions will be permanently stained black [21, 23]. This discoloration is caused by the oxidation of ionized silver into metallic silver [23]; this outcome limits the clinical use of silver compounds in aesthetically demanding patients. For the treatment of primary tooth that will exfoliate, the discoloration is not a long-term concern. Additionally, when applying this material, rubber dam or vaseline is recommended to avoid accidental contact with skin, gingiva, mucous tissue, and lips [23]. To address the esthetic issue, some clinicians have suggested placing a tooth-colored material such as glass ionomer cement on the stained and arrested caries lesion [21].

It is very important to inform and to explain to the patient these disadvantages before treatment to avoid dissatisfaction. A survey in New York, USA showed that staining on children’s posterior teeth was more acceptable to parents than staining on anterior teeth after caries treatment with silver compounds [49]. The survey also showed that parental acceptance increased in children who had more difficulties with receiving treatment. Parental acceptance of the treatment was related to socioeconomic status. Parents with a lower education level had higher acceptance and their acceptance level did not vary much as children’s difficulty with receiving treatment increased [49].

A temporary henna-appearing stain appears when SN or SDF solution comes into contact with skin. The skin pigmentation will disappear within 7 to 14 days, if the silver does not penetrate into the dermis [3, 21, 23].

In addition, unintentional placement of a high concentration of silver nitrate to the cornea might lead to blindness due to the fact that it can burn the cornea [24]. Therefore, patient and the operator must wear protective eye glasses. The solution should be always kept in small quantities. It is thus required to pay attention when handling silver solution during its application.

Nevertheless, exposure to a small amount of the solution will not produce immediate or even any side effects other than staining of the skin. Complications and side effects can become more noticeable with repeated exposures. The toxicity of SN and SDF is related to the dosage; however, the dosage used for treating dental caries is extremely small.