The Role of Fluoride on Caries Prevention

Function

How Does Fluoride Prevent Dental Caries?

Fluoride exerts its anti-cariogenic action when administered topically through three mechanisms: 1) it inhibits tooth demineralization, 2) it promotes tooth remineralization, and 3) it inhibits plaque bacteria.[8] Fluoride in drinking water and fluoride-containing products like toothpaste, mouthwash, and varnish work through these mechanisms. When fluoride is administered systemically in the form of supplements (drops, tablets, and lozenges), the effect on caries protection is minimal.[8]

Fluoride Inhibits Demineralization

As previously mentioned, fluoride incorporated into the enamel crystal structure at the tooth developmental stages does not have a sufficient effect on the acid solubility of enamel.[9][10] However, the fluoride present in the biofilm due to regular topical exposure and surrounding the carbonated apatite crystals (enamel crystals) effectively inhibits tooth demineralization.

At the time of bacterial acid production, the fluoride in the plaque fluid travels down with the acid into the enamel sub-surface, adsorbs into its crystal structure, and protects it against dissolution. Fluorapatite Ca10(PO4)6F2 is an enamel mineral much more resistant to acid dissolution than other tooth minerals (carbonated apatite and hydroxyapatite). Fluorapatite results from replacing the OH ion in hydroxyapatite with a fluoride ion.[8]

Fluoride Promotes Remineralization

During the normal process of remineralization, the acid produced by bacteria is neutralized by the saliva's buffering action, stopping the dissolution of enamel minerals at the subsurface.[8] Moreover, calcium and phosphate are supersaturated in saliva, forcing the minerals back into the dental structure. The partially demineralized crystals function as nuclei for further remineralization.[8]

Fluoride enhances tooth remineralization by accelerating the growth of fluorapatite crystals on the partially demineralized sub-surface crystals in the carious lesion. Fluoride adsorbs into this surface and attracts calcium ions. Consequently, this new surface attracts preferentially more fluoride, resulting in a fluorapatite-like surface.[8] Therefore, it is less soluble in plaque acids.

Fluoride Inhibits Plaque Bacteria 

Fluoride ions (F-) cannot cross the bacterial wall, but at low pH values (when bacteria produce acids), the fluoride ions (F-) travel through the cariogenic bacterial cell wall in the form of HF. [11][12] Once inside the cell, the HF dissociates again into H+ and F- ions, acidifying the bacterial environment and inhibiting the enzyme, enolase, and activity, respectively.[12] This process is cumulative.[8]

Clinical Significance

Community Water Fluoridation

Community water fluoridation has been used for over half a century, and results show that the greatest effect of fluoridated water on tooth protection is in deciduous and mixed dentition.[13][14][13] Water fluoridation is a simple, cost-effective method to prevent caries, and it is less than the cost of one dental restoration per person in their lifetime.[15] The United States Public Health Service recommends a fluoride concentration of 0.7 mg/L of water to prevent caries while reducing the risk of dental fluorosis.[16]

Recently there has been a rise in the consumption of bottled water that usually contains an insufficient concentration of fluoride.[17] In that case, alternative sources of fluoride supplementation should be considered. Also, over the decades, the protective benefits of water fluoridation have declined as other forms of fluoride delivery spread. For example, fluoride supplements are prescribed for children whose primary source of drinking water is deficient in fluoride.[13][18] 

It is highly recommended by the United States Preventive Services Task Force (USPSTF) that primary care providers (PCPs) prescribe fluoride supplements starting at six months of age to infants whose primary water source is lacking in fluoride.[18] Fluoride supplements include the systemic ingestion of fluoride in the form of dietary tablets, lozenges, or drops.

Dietary Flouride Supplements

Dietary fluoride supplements may be tablets, lozenges, or liquids such as fluoride-vitamin preparations.[19] Tablets and lozenges contain 1.0, 0.5, or 0.25 mg fluoride concentrations, and sodium fluoride is usually the active ingredient.[19] For infants, liquid fluoride supplements are used with a dropper. For children and adults, tablets are prescribed by dentists and PCPs, with instructions to chew or suck for 1 to 2 minutes before they are swallowed. This is to maximize the topical effect of fluoride.[20] One study showed an 80% reduction of caries in children who dissolved fluoride in the mouth by sucking or chewing than those who swallowed the fluoride tablets.

Fluoride supplements have been prescribed to compensate for fluoride-deficient water intake. Therefore, careful consideration must be given to the percentage of fluoride content in drinking water in the patient's region and any additional sources of fluoride the patient is receiving, such as toothpaste and mouth rinse.[19]

For children at high risk of caries living in areas where the drinking water contains less than 0.3 ppm F, daily supplementation of fluoride may be recommended as follows:

• From six months to three years of age: 0.25 mg of fluoride per day.
• From three to six years of age: 0.5 mg of fluoride per day.
• More than six years of age: 1.0 mg of fluoride per day.

Fluoride Toothpaste

Fluoride toothpaste delivers fluoride to dental surfaces, and it presents mainly as sodium fluoride (NaF). It has been effective in caries prevention of both primary and permanent teeth for decades.[21][22] Brushing with fluoride toothpaste increases the fluoride concentration in saliva by 100- to 1,000-fold. After one to two hours following using fluoride toothpaste, the saliva concentration returns to the baseline level.[23]

Toothbrushing must start when the first tooth erupts, usually as early as six months.[24] Brushing twice a day with fluoride toothpaste lowers the risk of dental caries by 14%.[25] Efficient plaque removal is attainable only while toothbrushing is supervised in children under four.[26]

Fluoride toothpaste containing 1,000 ppm of fluoride prevents dental caries in the permanent and primary dentition.[7] In the United States, the standard concentration of fluoride in toothpaste is 1,000 to 1,100 ppm.[19] Doses of 1,350 ppm to 1,500 ppm are advised for children at high risk of developing caries and seven years of age upwards.

In the early years of childhood, the swallowing reflex of children is not well controlled. For this reason, toothpaste containing 1,500 ppm fluoride is contraindicated in children under six to minimize the risk of fluorosis due to swallowing.[19] The recommended amount of toothpaste for toothbrushing according to age is only a smear of toothpaste in children less than three years and from three to six years of age a pea-sized amount.[27]

The maximum dose of fluoride toothpaste available over the counter in most countries is 1,500 ppm. Higher amounts (2,800 ppm and 5,000 ppm) are available on prescription. However, the indication of higher doses should be implemented as a short-term intervention to reduce risk in vulnerable young people and adults while other measures are being taken, like changing a cariogenic diet. On the other hand, patients at high chronic risk of dental caries could benefit from using such high doses for the long term, e.g., those with xerostomia or physical or learning disabilities. Sodium fluoride 2,800 ppm toothpaste can be indicated from 10 years of age and 5,000 ppm from 16 years of age.

Flouride Mouthwash

Flouride mouthwash helps prevent dental caries by increasing fluoride levels in dental plaque and saliva.[19] It is important that the mouthwash is used at a different time of the day than when toothbrushing to increase its availability in the biofilm and optimize its topical effect.[28]

The concentration depends on the frequency of use. Over-the-counter solutions of 230 ppm sodium fluoride are indicated for daily rinsing, and those containing 900 ppm for weekly use.[29] They are not recommended for children under six years due to the risk of swallowing – the risk/benefit ratio favors risk.[6] Parents must supervise the use of fluoride mouth rinses in permanent dentition as this has proven to be more efficacious than unsupervised utilization.

Fluoride Varnish, Gels, and Foams

The professional application of fluoride varnish (FV) and fluoride gels two or more times per year to children shows successful results in the prevention of caries in high-risk caries children of all ages regardless of the fluoride levels in drinking water. Fluoride varnish application twice a year reduces the incidence of caries by 37% in primary teeth and 43% in permanent teeth.[30]

Fluoride varnish, typically 22,600 ppm, is the only high-concentration fluoride formulation that can be used in children under six. It may be applied between twice or four times per year, according to caries risk. The United States Preventive Services Task Force (USPSTF) highly recommends that primary care clinicians apply fluoride varnish to the primary teeth of all infants and children starting at the age of primary tooth eruption.[18] In the UK, fluoride varnish is advocated for children over the age of 3 at risk of caries. The varnish must be placed as a thin layer on areas more at risk of caries, incipient lesions, and tooth defects.

Fluoride varnish is relatively safe but is contraindicated in individuals with a very rare allergic reaction to colophony (rosin), a substance in most varnishes.[31] It is also contraindicated in patients with contact dermatitis and stomatitis.

On the other hand, applying fluoride gels or foam twice a year (maximum application time is 4 minutes, and minimum application time is 1 minute) reduces caries incidence by 26% in permanent teeth of children who live in non-fluoridated areas. Fluoride gels are usually applied to teeth in the dental office in disposable trays to prevent dental caries and remineralize incipient lesions. They are available in 5,000 to 12,300 ppm. They are not recommended in children under six years of age due to the risk of swallowing the gel – the risk/benefit ratio favors risk.[6] They can be applied to children over six years twice to four times per year according to caries risk.

The American Dental Association (ADA) recommends 2.26% sodium fluoride varnish (22,600 ppm fluoride) for children under six years and 1.23% acidulated phosphate fluoride gel (12,300 ppm fluoride) for children over six years.[28] For home-use prescriptions, the ADA recommends a concentration of only 0.05% sodium fluoride gel (5,000 ppm fluoride) for children over six years or 0.15% stannous fluoride gel (1,000 ppm fluoride).[19] 

Silver Diamine Fluoride

Silver diamine fluoride (SDF), a colorless alkaline liquid, is regarded as the most cost-effective approach to dental caries prevention in low-socioeconomic settings and high-risk populations, including children and the elderly with special needs.[32][33] 

SDF works on two levels to arrest and prevent dental caries. The silver component has an antibacterial effect arresting carious lesions, and the fluoride component has a remineralizing action. However, the main drawback of using SDF is that it leaves a dark stain on the treated area, so it is used as a preventive measure where aesthetics is not the main concern.[34]

The use of silver diamine fluoride is a good solution to overcome barriers in dental treatment, such as dental fear, transportation, access to clinic facilities, insurance, and cost.[33] Comparisons of the annual application of SDF versus topical fluoride treatments, such as varnish and gels, placed two to four times per year show that SDF is more efficient.[35]

Slow-release Fluoride Beads

Slow-release fluoride beads are bonded to teeth to release low levels of fluoride in the oral cavity over a period of time, usually several years. Therefore, preventing dental caries and remineralizing incipient lesions.

The two main types of slow-release fluoride devices are copolymer membrane type and glass beads. Studies have shown that these devices raise intraoral fluoride concentrations enough to prevent enamel dissolution; however, further clinical trials are required to validate their use in clinical practice since the majority of research was done in vitro and in situ.[36] Also, although they may be a cost-effective solution in reducing dental caries in people at high risk of caries, their retention rates are the main drawback.[37]

Ecological Approaches To Caries Prevention

Probiotics

They are the 'good' bacteria that enhance the growth and survival of healthy oral microbiota. When probiotics are introduced to the oral biofilm, they create an imbalance in caries-related bacteria by disturbing its functional composition and metabolic activities, therefore, causing a shift in their local distribution.[38][39]

More research is needed to collect evidence on the effect of probiotic supplements on the diversity of the oral microbiome. Despite that, there are consistent results from clinical trials done on children under five years of age on the benefits of probiotic supplements to oral microbial diversity.[5] In addition, results from clinical trials have shown that school nutrition services that provided milk supplemented with probiotics and probiotic lozenges reduced caries development in preschool children and high caries-risk school children.[39]

Products That Modify The Oral Biofilm

The combination of fluoride and arginine cooperate in maintaining a healthy oral microbial balance. Studies have shown that oral hygiene products containing fluoride and arginine have greater anti-carcinogenicity than those containing only fluoride.[40]

Toothpaste Containing Enzymes And Proteins

Results of a clinical study show that toothpaste containing enzymes and proteins can increase the natural salivary defenses and shift the ecology of the oral microbiome to a healthier one.[41]

Other Issues

Fluorosis

Early childhood intake of fluoride supplements and fluoride levels greater than 0.7 ppm in drinking water are associated with the risk of mild to severe dental fluorosis with associated aesthetic concerns.[16][42] Ingesting greater than pea-sized amounts of toothpaste can lead to mild fluorosis in children younger than six.[43]

Children under four years of age are at risk of dental fluorosis in the permanent incisors and first molars since these teeth' calcification and maturation occur during these ages. Permanent premolars and second molars calcify and mature during four to six years of age; therefore, they are at risk of dental fluorosis. After six years of age, the risk of dental fluorosis is less significant.[6]

To avoid the risk of fluorosis in children and still reap the caries-preventive benefits of fluoride for all age groups, fluoride-containing products must be used appropriately.

Impact of COVID-19 on Preventive Care

The impact of the COVID-19 pandemic on preventive dental care was studied using fluoride varnish as the preventative measure in child wellness checkups. Results showed a significant drop in quarterly fluoride consumption and a decrease in routine dental visits. These findings highlight the importance of interprofessional collaboration between medical doctors and dentists to provide dental care in pediatric medical offices.[44] 

If dental care services continue to decrease due to the COVID-19 pandemic, access to dental care for children via non-traditional strategies is needed, such as teledentistry and preventive dental services in medical offices.[45]

Barriers to DCP Practice Measures

Barriers that deter dentists from using DCP measures include dentist beliefs, attitudes, and other factors. First, in most countries, dentists agreed that poor coping skills of young children during their dental visits limit DCPs from delivering appropriate dental care and, therefore, DCP measures.[46] Second, dentists' attitudes include negative personal feelings, such as feeling time-constrained in the practice setting and that providing dental services can be stressful.[46] Third, factors with the highest frequency effect size acting as barriers to dentists practicing preventive dental care are low compensation (25%), length of time since graduation (22%), and male dentists (19%). 

On the other hand, the main reasons that encouraged dentists to adhere to preventive measures were teamwork (21%), post-graduation education (12%), and professional understanding of the benefits of DCP (12%).[47] This highlights the importance of post-graduate, interprofessional DCP education.

Enhancing Healthcare Team Outcomes

Although oral health is part of the overall systemic health, oral health care is not the main target of pediatric care providers. Collaborative efforts among health professionals are paramount to prevent the recurrence of dental caries.[48] Primary care providers play an important role in the oral health of young children. Pediatricians and family physicians provide immunizations and routine child visits; they are the first to interact with caregivers of children under five years of age.[44][48] 

A recent review of oral interventions for pregnant women and young mothers showed that healthcare providers could help reduce the incidence of early childhood caries by delivering oral health education to expecting mothers, conducting caries risk assessments, and referring their patients to the dentist.[49]

According to a survey, most pediatric care providers will only refer a child with high caries risk to the dentist, but a few will refer a child for routine dental visits.[50] In another review, it was found that often,  pediatricians have little knowledge of DCP strategies in young children. Possible reasons include limited education and training, time constraints, and a lack of clear referral pathways. Dental teams can work on ensuring clear and easy referral pathways from primary healthcare teams and non-dental health professionals.[44] 

Inter-professional education in graduate programs has improved knowledge, confidence, and practice toward children's oral health.[48]

Healthcare providers other than dentists can prevent dental caries if appropriately trained. Primary care clinicians and pediatricians can also detect early caries lesions, which appear as white spots on the gingival margin or brown stains on the occlusal surfaces, and make a prompt referral to the dentist.[4] 

The professional application of topical fluoride (varnish and gel) can be provided by medical professionals to children at high risk of caries. Oral health can be significantly improved by training a more comprehensive range of people to advise on dental caries prevention and provide preventive interventions, e.g., fluoride varnishes, including a mix of healthcare providers, nurses, and people from the community and schoolteachers.[4] Therefore, prompt consultation with an interprofessional group of specialists is recommended to improve outcomes.