Caries–An Ongoing Public Health Crisis

Dental caries continues to be a globally widespread disease and an ongoing public health crisis, creating a significant financial and social burden on a worldwide scale. (The articles in this series were presented at a symposium and did not undergo peer review.)

ABSTRACT
Despite being a preventable disease, dental caries remains a global health crisis. Our understanding of the caries process has changed over the last few years; it has evolved from being a unidirectional process resulting in cavitation to a dynamic process of demineralization and remineralization. An examination of the impact of caries as a global crisis demonstrates that caries continues to be a chronic problem despite being a preventable and treatable disease. The impact of caries is seen in the health and wellbeing of the population, as well as the economy. Current preventative methodologies have focused on the utilization of fluoride to increase the remineralization process and reduce demineralization. Fluoride has a wide range of applications, from at-home techniques such as fluoridated toothpaste and mouth rinses to community-level approaches such as public water fluoridation. Additional management methodologies have emerged; silver diamine fluoride has only recently been introduced in countries such as the United States despite being used for decades as a caries-arresting material in Asian countries such as Japan. The benefits of such a medicament have been crucial in approaching caries control in underserved populations as well as in aging and special health care needs patients. Dental technology continues to improve, and the latest focus has been on a new biofilm-modifying preventative alternative: arginine toothpaste. The incorporation of arginine in the biofilm is believed to create a probiotic/prebiotic effect in which commensal bacteria create a more alkaline environment, favoring a reduced acidic environment that causes tooth demineralization. Fortunately, once caries demineralization has extended beyond the stage at which remineralization can restore surface integrity, the carious tooth can still be managed without surgical restoration by caries arrest.

INTRODUCTION
To best understand the “ongoing public health crisis” of caries, it is necessary to discuss the evolving definition of dental caries. Although the current definition of dental caries is accurate, the localized destruction of susceptible dental hard tissue by acidic byproducts from bacterial fermentation of dietary carbohydrates,¹ this definition has expanded to a more complex discussion of the caries process that represents a continuum of tooth demineralization/remineralization. The “modern” view of dental caries started with W.D. Miller’s (1890) definition of a 2-step process whereby bacteria on the tooth, exposed to fermentable carbohydrates, produce acid and in a second step dissolve the surface of the tooth.² Stephan demonstrated that this production of acid after exposure to the fermentable carbohydrate resulted in a localized drop in pH within the plaque, followed by a subsequent return to the baseline pH over time.^3,4 This set the stage for caries being a cyclic event of demineralization/neutrality/remineralization. Englander (1959) demonstrated the role of saliva in neutralizing the fall in plaque pH after exposure to fermentable carbohydrate.^5,6 Although considerable attention has been placed on a few bacterial species as the cause of dental caries (eg, Streptococcus mutans, Lactobacillus…), there is general agreement today that the dental biofilm exists as a complex ecosystem that can shift from a neutral pH to a more acidic ecosystem.^7,8 Today, dental caries is accepted as an imbalance in the cyclic process of demineralization and remineralization of tooth structure. This is caused by a mixed ecology biofilm consuming fermentable carbohydrate excreting acids resulting in a pH maximal drop followed by the return of the pH to initial pH modulated by saliva. There is currently a recognition that the mixed ecology of the biofilm may be naturally shifted in composition to a more acidic ecology by repeated exposure to fermentable carbohydrate.^7,8 Saliva helps modulate both the composition of the biofilm⁷ and the recovery of the biofilm pH after sugar challenge.⁵ The susceptibility of the tooth to demineralization may be modulated by the incorporation of fluoride in the tooth structure.⁹ Dental caries is a continuum that involves the balance between the time a tooth surface spends in demineralization versus the time remineralizing. An imbalance in the continuum resulting in greater time spent in demineralization will lead to tooth surface alterations that eventually result in cavitation. There are a number of items that influence the time a tooth spends in demineralization. The increase in frequency of sugar consumption and an increase in sugar retention time directly relate to an increase in demineralization of teeth resulting in cavitation, measured clinically as decay/missing/filled/teeth (DMFT),¹⁰ but it must be remembered that in this balance, frequent/longer acid cycles result in a shift in the biofilm flora in favor of acidogenic bacteria.⁸ Importantly, an increase in the quantity of sugar consumed alone is not a predictor of increased caries, as many communities around the world have increased caries preventive strategies over the same time period as sugar consumption has increased.¹¹ Decreases in saliva flow can also favor a shift in the caries balance toward demineralization, resulting in an increase in caries progression.^12,13 The caries balance is affected by multiple social determinants favoring demineralization that will be discussed later. Research has demonstrated that acid is not the only product of the mixed ecology. Alkali production has potential in changing the pH of the oral biofilm, which impacts demineralization.¹⁴ Recent dental research has focused on the ability of an amino acid, arginine, within toothpaste to complement the anti-caries effect of fluoride. By including arginine in toothpaste as a prebiotic, microorganisms commensal in the biofilm create a more alkaline environment that helps to reduce the acidity that causes enamel demineralization and caries formation in the first place.¹⁵ 

Dental caries is a preventable disease, yet it consistently ranks as one of the most common diseases affecting the worldwide population.¹⁶ One of the more evolutionary changes in the understanding of caries is that caries is a lifelong disease.¹⁷ Increases in the number of untreated carious teeth are seen in an aging population worldwide as individuals live longer with their teeth. Although traditional dental care has focused on repairing decayed teeth to restore form and function, the last few decades have attempted to develop a more preventative approach to caries management. Fluoride has been shown to be one of the primary methods available to combat the effects of demineralization of teeth. Today, fluoride is utilized in community water, fluoride toothpaste, silver diamine fluoride, and high-concentration fluoride supplements to manage caries.

Caries as Global Health Problem
Dental caries remains a global health problem and is consistently recognized as a highly prevalent yet neglected health concern.¹⁸ The Lancet Commission on Oral Health (2019),¹⁹ a recent World Health Assembly Resolution,²⁰ and Vision 2030: Delivering Optimal Oral Health for All²¹ each point to the ongoing nature of the global caries crisis. Across the globe, the number of people who have some type of untreated oral condition (primarily untreated caries, severe periodontitis, and severe tooth loss) has been on the rise. In the Global Burden of Disease 2017 review, 3.5 billion people worldwide have been afflicted by oral disease; of this number, 2.3 billion had some form of untreated dental decay.²² Although caries was found to be ubiquitous throughout the world, the WHO Global Oral Data Bank survey of 12-year-olds found that reported prevalence ranged from 90% to 20%, with DMFT ranging from 0.3 to 6.1.²³ The disparity within countries and between countries was not always explained by socioeconomic status, access to care, and rural versus urban communities. The burden of tooth decay for 12-year-olds was highest in middle-income countries, with about two-thirds of decay remaining untreated.²⁴ A review of studies in the Africa and Middle East Region found that caries prevalence and DMFT was greater in urban versus rural for these countries, with the daily consumption of sugar in urban areas growing more rapidly than rural communities and the daily consumption of fruits greater for rural areas than urban in the same countries.²⁵ There has been a significant shift in burden of untreated caries globally, and, in high-income countries, the shift from children to adults challenges the convention that lower levels of caries in childhood will continue throughout a lifetime, with the concomitant cost of care rising steeply.^16,26

The prevalence of caries and tooth loss due to caries in the United States has been essentially decreasing over the last half-century, but the data clearly demonstrate that there are social disparities in how well the United States is managing dental caries.¹⁷ Unfortunately, there is a racial and ethnic disparity when evaluating oral health. The Surgeon General’s report on Oral Health in America recognizes that caries is a disease of disparity, and it is important to acknowledge that it does not affect all Americans equally. According to the CDC, “Non-Hispanic blacks, Hispanics, and American Indians and Alaska Natives have the poorest oral health of any racial and ethnic groups in the United States.”¹³ Adults (35 to 44 years) who identify as Black, Non-Hispanic, and Mexican Americans have double the amount of untreated tooth decay compared with their non-Hispanic White counterparts. This disparity continues in Mexican American and non-Hispanic Black children having more tooth decay in the 3 to 5 and 6 to 9 age groups than their peers.¹³ Based on these reports, we see that specific racial groups suffer a disproportionate burden for dental caries. 

Dental caries also affect poor families to a greater extent. Children aged 5 to 19 from a low-income family (<100% of the federal poverty level) have at least double the amount of tooth decay compared with children from high-income families (>200% of the federal poverty level). Despite this, there is hope. Studies have evaluated poverty status and compared data from the periods of 1988 to 1994 and 1999 to 2004 demonstrating that although children from low-income families had more tooth decay in general, children from high-income families saw an increase in dental caries over the same time period, suggesting that the strategies used in low-income families, such as sealants, are helping the lower-income population.¹⁷

Economic and Social Impact of Dental Caries
In children, dental caries has been recognized as the single most common chronic disease. Dental caries is 5 times more common than asthma and affects 1 in 5 children in the United States.¹³ Unfortunately, because of a lack of public awareness, dental caries remains a prevalent yet preventable disease, especially in this age group. Dental caries can have an impact on child development and learning. Pain from untreated decay can cause a child to lose the ability or desire to eat, resulting in deficient nutritional intake with consequent systemic effects.²⁷ Additionally, having pain or concerns in the oral cavity can also affect a child’s speech, self-esteem, and overall quality of life.²⁸ Children with active caries have been shown to miss more school days and, in general, perform worse than their peers.¹³ These students also have more difficulty concentrating on their school work and tend to be more irritable because of their oral pain. It is estimated that every year, over 51 million hours of school are lost because of some dental-related cause.²⁹ Poor childhood caries control is a concern because caries of the primary dentition is a significant predictor of future decay in the permanent dentition. Combined with the fact that oral hygiene declines during the adolescent years, it is understandable that these patients will continue to be at high risk as they enter into adulthood.³⁰

Adults actually experience more decay than children—1 in 4 adults have untreated decay.²⁹ Like children, adults miss a significant amount of time from work because of dental disease or visits. It is estimated adults miss 164 million hours a year of work because of oral health issues.¹³ Dental emergencies are predicted to result in a $45 billion loss in productivity due to palliative or emergent care.²⁹ The high rate of decay in the overall population has also caused an increase in worldwide dental costs, which are estimated to reach over $544.41 billion US dollars in 2015.³¹ Dental costs can be divided into 2 categories: Direct cost is the actual expenditure for care, whereas indirect cost is the loss in productivity due to disease. Regionally, high-income North America ranks at the top for direct costs around the world and is a close second for indirect costs. On an individual country level, the United States spends the most money for dental care and also loses the most money indirectly to this disease.³¹ Considering the preventability of dental caries, there should be an increase in investment in the oral health of the American population. 

Fluoride—Preventing Demineralization and Encouraging Remineralization
Fluoride, delivered through a variety of methods (community administered, home-administered fluoride toothpastes, and high-concentration fluorides), is a substantial factor in the reduction of caries burden over the last 75 years.¹³ Although the systemic action of fluoride appears to yield significant protection when other sources of fluoride are not available, the topical effect of fluoride is recognized as being crucial. Toothbrushing alone can remove plaque from the tooth surface, preventing further demineralization, but the added effect of fluoride from toothpaste aids in the remineralization process, and fluoride can be incorporated into the tooth structure as fluorapatite, which builds resistance against an acidic environment. The introduction of a preventive brushing and fluoride program in the early 1980s in Germany decreased the number of teeth with decay in 12-year-olds by 93% and halved adult tooth loss between 1997 and 2014.²⁶

Community fluoride administration includes milk, salt, and public water supply. Fluoride has also been incorporated into drinking water since 1945, when fluoride was first added to the water in Grand Rapids, Michigan; as of 2012, about 200 million people in the United States have benefitted from water fluoridation, especially those in underserved communities, those with disabilities, the aging, and those with economic challenges.^32,33 Water fluoridation has been acknowledged as one of the greatest public health achievements of the United States in the 20th century. It is safe, efficacious in preventing tooth decay, and generally considered an inexpensive cost per person for a community. One major negative health effect on the dentition associated with community water fluoridation and consumption of other sources of fluoride is fluorosis. Fluorosis may present in many different forms, ranging from the mild form in which white discolorations appear on the tooth surface to severe fluorosis in which the teeth can appear pitted or brown. The United States’ population is at high risk for fluorosis because of the availability of multiple fluoride sources. The National Health and Nutrition Examination Survey data from 1999 to 2004 found 23% of Americans aged 6 to 49 were affected.³⁴ The majority of this population was afflicted by a mild form of fluorosis, and fewer than 1% experienced the severe form. 

Fluoride-containing toothpastes have repeatedly demonstrated significant caries reductions if utilized once per day regardless of fluoridated water consumption, and greater improvements have been seen when utilized twice a day.³⁵ The use of additional topical fluoride, gels, rinses, and varnishes demonstrated additional caries preventive fraction.³⁶ Professionally applied fluoride varnish applications 2 to 4 times per year have demonstrated significant reductions in new caries for children and adolescents.^37,38 Fluoride has clearly played a significant role in the prevention of new dental caries over the last 6 to 7 decades.

Strategies for management of the biofilm for caries prevention
Management of the oral biofilm has been a cornerstone of dental health. Multiple investigations have been conducted to make the biofilm less cariogenic by regulating S. mutans through novel molecules, peptides, and sugars with little success in actual caries reductions.^39-43 Attempts to alter the biofilm microenvironment have offered hope for a new pathway to reduce caries. The naturally occurring amino acid arginine was found to be metabolized by a number of commensal bacteria such as Streptococcus sanguinis in the biofilm and produce ammonia, which is basic.^44-47 Studies have demonstrated that patients brushing with a 1.5% arginine-containing toothpaste develop higher resting plaque pH and a plaque that does not have as low a terminal pH when challenged by sugar. This decreases the interval below the critical pH that most consider the time spent demineralizing the tooth surface. This higher pH is primarily the effect of arginine metabolism and the production of ammonia, as well as lower levels of lactic acid.^45,47 Multiple clinical studies have demonstrated improved increment of caries prevention and early caries lesion remineralization.^47-50 These clinical studies demonstrate both a prevention of new decay and remineralization of early lesions utilizing quantitative light-induced fluorescence, as well as reduction of DMFT in long-term clinical trials.

Management of the caries lesion
Restoration of teeth does not treat the caries process. The need for prevention, early diagnosis, and management in the early stages of caries is critical.⁵¹ Early noncavitated lesions may be treated utilizing highly conservative nonsurgical techniques that restore the tooth to health. Early lesions are considered those with intact or nearly intact enamel surface, where demineralization may be evidenced penetrating as far as the dentin. These lesions may be inactive, with a glossy smooth surface, or active, with a frosty roughened surface. Where inactive lesions may be observed for future changes in surface characteristics, active lesions require intervention both in risk reduction and in the attempt to remineralize.

Remineralization/Arresting Caries
Remineralization is the process by which the tooth gains calcium into the enamel and/or dentin that had previously been demineralized in the caries process. Remineralizing an active caries lesion increases the mineral content of the tooth previously damaged by decay and, when occurring in a noncavitated early lesion, may actually restore the tooth’s surface to a smooth surface. Arresting the caries process results in no further net loss of mineral from previously active caries lesions.¹ Essentially, arresting caries stops the biologic caries process from advancing and may or may not restore the tooth surface depending on the amount of pre-existing tooth destruction.

Two major therapeutics have emerged to manage the remineralization and/or arrest of caries: the utilization of fluoride on active caries lesions and the use of silver diamine fluoride (SDF). For noncavitated active lesions, sodium fluoride (5%) applied every 3 to 6 months has demonstrated the ability to arrest and remineralize coronal carious tooth structure, even without additional homecare or dietary modification.^52-54 Sodium fluoride (0.2%) mouth rinses utilized once per week and acidulated phosphate fluoride gel (1.23%) professionally applied 4 times per year have both demonstrated moderate certainty to arrest or reverse noncavitated lesions.⁵³ Cavitated lesions on all surfaces except root surfaces are less likely to arrest with the above fluoride alone than with SDF.⁵³ SDF, originally utilized in Japan in the 1960s to prevent caries, was officially introduced to the United States in 2014 as 38% SDF (with 5% fluoride) and as a means to reduce dentin sensitivity. Dentists have been using SDF (off label) to arrest carious lesions in the primary and permanent dentition. It is believed that the silver component of SDF has an antimicrobial effect, specifically inhibiting S mutans and Lactobacillus acidophilus, whereas fluoride is able to prevent further decay through remineralization because SDF is able to form CaF2, which leads to the formation of fluorapatite.^55-57 Application of SDF is a quick procedure and easily completed with simple cotton roll isolation. SDF has been demonstrated to be highly effective in arresting decay and can penetrate the enamel to a depth of 25 μm and the dentin to 50 to 200 μm.⁵⁸ With regular applications twice a year, active caries are arrested and the incidence of new caries is reduced.⁵⁵ Biannual utilization of 38% SDF has demonstrated a 74% caries arrest rate.⁵⁹ When compared with fluoride varnish or glass ionomer, SDF is significantly more effective.^53,60 The concerns for utilization of SDF have primarily been regarding the side effect of tooth discoloration. Any decalcification or carious lesions may turn a dark brown or black color because of the silver phosphate being photosensitive.⁵⁹ In a study assessing parents’ perception of SDF treated teeth, the majority of parents were not concerned by the discoloration to their children’s teeth.^{61, 62} 

Despite the success of SDF in arresting carious lesions, SDF does not rebuild tooth structure in gross lesions. Other techniques, such as the atraumatic restorative treatment (ART), may be utilized in conjunction with SDF to form the silver-modified ART or an ART restoration on its own. ART is a technique that can be performed without local anesthetic or rotary instruments, removing soft caries, finding caries-free margins, and placing a glass ionomer restoration to restore the tooth surface.⁶³ Adding a glass ionomer or resin-based glass ionomer over a frank cavitation protects the surface of the lesion and restores form; however, this technique is more successful for class 1 lesions than class 2 lesions.⁶⁴ ART restorations alone have demonstrated the ability to arrest caries and have similar long-term performance as amalgam restorations.⁶⁵

Other methods, such as resin infiltration on smooth surfaces and sealants in pits and fissures, have demonstrated the ability to arrest cavitated and noncavitated dental caries, frequently in concert with 5% sodium fluoride varnish, although the evidence supporting the use of both therapies has a low certainty.⁵⁴ There is also weak evidence for the effectiveness of 1% chlorhexidine/1% thymol varnish on arresting root caries.^53,66 SDF continues to be more successful in arresting decay and has seen increased utilization across the United States.

Conclusion
Dental caries continues to be a globally widespread disease, creating a significant financial and social burden on a worldwide scale. Our understanding of the dental disease has been refined over the years. As we learn more about the dental biofilm and its characteristics, dentistry is able to develop new methods to disrupt the biofilm, prevent initial caries, or arrest and reverse current lesions.