Dental caries is a biofilm-mediated, sugar-driven, multifactorial, dynamic disease that results in the phasic demineralization and remineralization of dental hard tissues. Caries can occur throughout life, both in primary and permanent dentitions, and can damage the tooth crown and, in later life, exposed root surfaces. The balance between pathological and protective factors influences the initiation and progression of caries. This interplay between factors underpins the classification of individuals and groups into caries risk categories, allowing an increasingly tailored approach to care. Dental caries is an unevenly distributed, preventable disease with considerable economic and quality-of-life burdens. The daily use of fluoride toothpaste is seen as the main reason for the overall decline of caries worldwide over recent decades. This Primer aims to provide a global overview of caries, acknowledging the historical era dominated by restoration of tooth decay by surgical means, but focuses on current, progressive and more holistic long-term, patient-centred, tooth-preserving preventive care.
Dental caries involves interactions between the tooth structure, the microbial biofilm formed on the tooth surface (Fig. 1) and sugars, as well as salivary and genetic influences1. The dynamic caries process consists of rapidly alternating periods of tooth demineralization and remineralization, which, if net demineralization occurs over sufficient time, results in the initiation of specific caries lesions at certain anatomical predilection sites on the teeth. It is important to balance the pathological and protective factors that influence the initiation and progression of dental caries. Protective factors promote remineralization and lesion arrest, whereas pathological factors shift the balance in the direction of dental caries and disease progression1 (Fig. 2). The daily use of fluoride toothpaste is seen by many authorities as the main reason for the overall decline of caries worldwide over recent decades; the mode of action of such toothpastes is concerned with shifting the balance of the oral biofilm towards health.
There is not a direct correlation between the extent of a caries lesion and whether pain and discomfort is felt. However, severe toothache, when it occurs, can be disabling, and infection and sepsis arising owing to caries that spreads to involve the dental pulp can occasionally lead to serious systemic consequences, such as spreading local infection and, very rarely, treatment-related death (as a complication of anaesthesia), as well as to tooth loss.
The clinical detection of caries is traditionally made by detailed visual inspection of clean teeth by trained examiners. Although sharp pointed dental probes (or explorers) are still often used, they provide little additional diagnostic benefit and can do some damage. Dental radiographs or other supportive diagnostic methods are also needed in clinical practice to detect lesions that are hidden to visual assessment, particularly those situated on the approximal tooth surfaces (that is, surfaces that form contacts between adjacent teeth).
Although the ravages of caries can make teeth appear to be highly vulnerable to destruction by disease, from an evolutionary biology perspective, human teeth are a high-valued organ system involved in the prehension and processing of food, and can also function in defence, sexual attraction and phonetic articulation2. The outer surface of the tooth crown is composed of enamel, the hardest substance in the body (Fig. 1), with saliva, a specialized fluid, being secreted throughout the day to preserve its integrity. The morphology of the modern dentition has evolved mainly based on our dietary preferences, which have changed over the millennia2. Interestingly, diets high in sugar tend to be soft and often liquid; teeth are not required for their ingestion, which might explain why teeth can be rapidly lost.
This Primer aims to provide a balanced international overview of dental caries, both as a complex, multifactorial disease and as a dynamically fluctuating disease process. The article covers the full range of perspectives from epidemiology to quality of life — via pathophysiology, diagnosis, risk assessment and prevention. Public health aspects are an important complement but are not covered in depth for reasons of space. Current research evidence is helping to chart the way to a more biologically based way of planning and delivering caries prevention and care, at both the population and the individual levels. These considerations underpin the development of science, practice and policy to optimize patient care and health.
Epidemiological studies of caries have been undertaken for many decades, and some of the data available through the WHO and other organizations give an impression that we have plentiful comparable global data. However, to evaluate and plan policy, epidemiology should provide data that meet the following specification: timely, accurate and understandable data for key age groups on the total amount of disease present (prevalence), the rate of disease progression (incidence) and disease trends over time. In addition, information on variations in disease levels between and within countries, including the estimates and trends in health inequalities (that is, differences in health status between groups within populations) are needed. However, we do not currently have accurate, up-to-date, clinically meaningful information across the globe that meets these specifications. Dental caries is still a neglected topic, despite the acknowledgment of the WHO that is still a major health problem in most industrialized countries, in which 60–90% of children and the vast majority of adults are affected by dental caries3. Although caries has been considered a childhood disease, in reality, it continues into adulthood4. Health inequalities exist in the burden of dental caries in both children and adults5.
Dental caries is considered to be the single most common chronic childhood disease, and its prevalence is thought to have increased recently in children 2–5 years of age worldwide, making this age group a global priority action area6,
Traditionally, low caries prevalence has been observed in the developing countries, whereas the prevalence is higher in developed countries3. This geographical situation has become more complex owing to speed of economic development and rapid changes in habits and diet in many countries. Although there might be sex or ethnic differences, they are minor compared with factors such as sugar consumption, lifestyle and economic differences.
The traditional global index used to measure caries in epidemiological studies — but not in clinical practice — is the DMF (Decayed, Missing and Filled) Index, which is a numerical count of affected teeth per individual collected at either the Tooth (DMFT) or tooth Surface level (DMFS). The count of DMFT for an individual or group records their caries experience (that is, the total of both current and past caries). The index can be used at the different diagnostic thresholds, which affect both the mean DMFT and the proportion of individuals affected11,
Efforts are under way worldwide to improve our understanding of caries epidemiology by improving methodologies and optimizing them for use in epidemiological field work, while also keeping compatibility with systems used in a fully equipped dental practice. In epidemiological studies, the International Caries Detection and Assessment System (ICDAS) ‘epi-modifications’ (Ref. 12) (Fig. 3), which can be used alongside the WHO basic reporting criteria13, have now been used in many countries15, along with the more-recent simplified merged codes option16. The merged ICDAS codes, which are closer to those used in clinical practice, considers sound surfaces and three stages of caries (as opposed to sound surfaces and six stages of caries in the full codes ICDAS option). It is also possible to combine clinical and radiographic findings to reveal the full prevalence of caries17. Recent work by several European organizations has shown — by way of a global example — that most of the current national caries data for DMFT levels in children 12 years of age are not comparable across Europe15. This highlights the real challenges facing epidemiological studies on caries, whereby apparently comparable results from across several countries have, in fact, been collected at different time-points with very variable levels of training and calibration and, thus, record caries at different thresholds.
When oral health topics were added to the ongoing Global Burden of Disease Study18, oral diseases were found to be highly prevalent, affecting approximately 3.9 billion people worldwide. The methodology used in this major study is useful because it allows comparison with other diseases in terms of burden, but it is also novel in terms of caries epidemiology as it does not use the DMF Index (which has been used globally for the past 60 years). Untreated caries in permanent teeth was the most prevalent condition evaluated across all medical conditions, with a global prevalence of 35% for all ages combined, with 2.4 billion people affected. Note that some of these permanent teeth will have been in children and adolescents. Untreated caries in primary teeth in children ranked 10th in prevalence, affecting 621 million children worldwide.
The mechanisms and pathophysiology underlying the development of dental caries are now increasingly well understood and are best considered first from the hard tissue-related aspects (as the disease affects the calcified dental tissues) and then from the microbiology (biofilm)-related aspects (as these represent the driver of the caries process if homeostatic imbalance is maintained) (Fig. 1). However, because of the multifaceted nature of the disease process, these factors are not independent. The dental hard tissues that are exposed to the oral environment (crowns and, later, roots following gingival recession) are the targets of the caries disease process, and all tooth surfaces are susceptible throughout an individual's lifetime. However, caries will not occur in the absence of a cariogenic (that is, pathogenic) dental biofilm and frequent exposure to dietary carbohydrates, mainly free sugars19,20, and therefore caries must be considered a dietary–microbial disease21. A modern concept of caries also includes consideration of how behavioural, social and psychological factors, as well as biological factors, are involved22,
Demineralization and remineralization
Dental caries typically start at and below the enamel surface (the initial demineralization is subsurface), and is the result of a process in which the crystalline mineral structure of the tooth is demineralized by organic acids produced by biofilm bacteria from the metabolism of dietary fermentable carbohydrates, primarily sugars. Although a wide range of organic acids can be generated by dental biofilm microorganisms, lactic acid is the predominant end-product from sugar metabolism26 and is considered to be the main acid involved in caries formation. As acids build-up in the fluid phase of the biofilm, the pH drops to the point at which conditions at the biofilm–enamel interface become undersaturated, and acid partially demineralizes the surface layer of the tooth27. The loss of mineral leads to increased porosity, widening of the spaces between the enamel crystals and softening of the surface, which allows the acids to diffuse deeper into the tooth resulting in demineralization of the mineral below the surface (subsurface demineralization). The build-up of reaction products, mainly calcium and phosphate, from dissolution of the surface and subsurface raise the degree of saturation and can partially protect the surface layer from further demineralization. In addition, the presence of fluoride can inhibit the demineralization of the surface layer28. Once sugars are cleared from the mouth by swallowing and salivary dilution, the biofilm acids can be neutralized by the buffering action of saliva. The pH of biofilm fluid returns towards neutrality and becomes sufficiently saturated with calcium, phosphate and fluoride ions so that demineralization stops and re-deposition of mineral (remineralization) is favoured. Owing to the dynamic nature of the disease process, the very early (subclinical) stages of caries can be reversed or arrested especially in the presence of fluoride.
As demineralization progresses into the subsurface of the enamel and dentine in the case of root caries, with a continuing acid challenge and pH drop, the rate of mineral loss becomes greater in the subsurface than at the surface, resulting in the formation of a subsurface lesion. When sufficient mineral is lost, the lesion appears clinically as a white spot. This is a clinically important stage of the caries process, as the lesion can be arrested or reversed by modifying the causative factors or applying preventive measures; however, the repair process is typically mostly restricted to the surface layer.
At this stage in development, initial-stage caries (ICDAS codes 1 and 2) are considerably demineralized. With changes in the local ecology, dietary practices and fluoride availability they may: arrest and remain as inactive lesions that do not progress but can still be detected as a scar because of the changes in the optical properties of the enamel; remineralize and effectively heal, whereby re-precipitation of mineral in the lesion and possibly some superficial surface wear results in an apparently sound surface; or remain active and progress to a more extensive stage of destruction.
If the caries process progresses further, the surface porosity increases with the formation of microcavitations in the enamel (ICDAS code 3) or, in root caries, a progressive softening of the surface dentine layer. In caries of the tooth crown, the surface layer of the lesion may eventually collapse, resulting in physical cavitation (a macroscopic hole — ICDAS code 5 or 6). Even at this more extensive stage of caries severity, a lesion may in optimal circumstances still arrest, although the biofilm-retaining cavity will persist. When an irreversible stage of lesion extent is reached (typically, in most developed countries, at ICDAS codes 5 and 6), combined with symptoms and/or considerations of the functional or aesthetic needs of the patient, operative intervention is indicated. If the caries process continues, eventually the dental pulp will be compromised and either a root canal treatment or tooth extraction will be necessary.
For optimal tooth health, the main goal is to maintain the mineral homeostasis of tooth surfaces. As teeth are frequently exposed to acidic conditions either from biofilm or dietary acids, the ability to remineralize is essential to maintaining tooth integrity. Saliva is essential for the preservation of tooth health by providing the minerals necessary for remineralization. Low levels of fluoride greatly enhance this process, which largely explains the remarkable effectiveness of fluoride in several delivery forms in reducing dental caries28 (Box 1).
Dental caries is a dynamic disease process that involves repeated cycles of demineralization and remineralization throughout the day27,29. Teeth are most susceptible to caries when they first erupt in the mouth and over time become more resistant to subsequent acid challenge. The clinical implication is that there should be greater focus on monitoring the caries status of teeth and delivering preventive care during the periods when teeth are erupting.
Microbiology and dental biofilms
Oral microbiota in health. The mouth, like other surfaces of the body, is colonized from birth by a diverse array of microorganisms (collectively known as the oral microbiota)30. The most common group of microorganisms is bacteria, but yeasts, viruses, mycoplasmas, protozoa and Archaea can be present. The oral microbiota has a symbiotic or mutualistic relationship with the host. The resident oral microorganisms benefit from a warm and nutritious habitat provided by the host and, in return, act to repel invading microorganisms, contribute to the host's defences and engage in crosstalk with the host to downregulate potentially excessive pro-inflammatory responses to commensal bacteria31. Saliva has a crucial role in maintaining this beneficial microbiota by buffering the oral environment at a neutral pH (which is optimal for the growth and metabolism of most of the oral microbiota), while providing proteins and glycoproteins as nutrients.
Dental biofilms. The oral microbiota grows on surfaces as structurally and functionally organized communities of interacting species, termed dental plaque32,33. Dental plaque is an example of a biofilm, the formation of which involves several stages34. Tooth surfaces are covered by a conditioning film of proteins and glycoproteins (the acquired pellicle) that are derived mainly from saliva, but it also contains bacterial components and their products, gingival crevicular fluid (that seeps from the junction between the gum and the tooth), blood and food35 (Fig. 1). The acquired pellicle provides binding sites for adherence by early bacterial colonizers of the tooth surface, leading to dental biofilm formation, and acts as a physical barrier preventing acid diffusion36.
Bacteria can be held weakly and reversibly near the surface by long-range van der Waal forces (forces that do not involve covalent or ionic bonds) between the external layers of the bacterium and this conditioning film. Attachment becomes stronger and more permanent if interactions occur between molecules on the bacterium (adhesins) and complementary receptors in the conditioning film32. Secondary colonizing species attach to the early colonizers (co-adhesion), and the complexity of the biofilm increases. The biofilm undergoes maturation, and numerous synergistic and antagonistic microbial interactions occur37. A matrix is formed, which is composed of bacterial exopolymers (polymers secreted in the external environment), including polysaccharides derived from sugar metabolism and DNA; the matrix helps to retain the biofilm on the surface and can influence the penetration and movement of molecules within the biofilm37,38. The biofilm protects the bacteria against antimicrobial agents. The composition of these biofilms varies on different surfaces of the tooth owing to subtle differences in the local environmental conditions.
Microbial aetiology of dental caries. The normally synergistic relationship between the resident microbiota and the host is dynamic and can be perturbed by changes in lifestyle or alterations to the biology of the mouth; these changes can predispose sites to disease. Risk factors for caries include the frequent consumption of fermentable dietary carbohydrates (especially sucrose)39 and/or a reduced saliva flow40. Numerous cross-sectional and longitudinal epidemiological studies have reported a shift in the balance of the microbiota at sites with caries compared with sites with sound surfaces. Early studies of caries lesions found higher proportions and incidence of Streptococcus mutans and Streptococcus sobrinus than sound enamel; lactobacilli were isolated from advanced lesions40. These observations led to the proposal that caries is only caused by a limited subset of the many species found in dental biofilms (the ‘specific plaque hypothesis’)41. However, as more epidemiological studies were performed, caries were observed in the apparent absence of these bacteria, whereas these organisms could persist on other surfaces that remained sound.
Subsequent laboratory studies confirmed that other bacteria found within dental biofilms could also generate a low pH from sugars, whereas others could reduce the potentially damaging effect of lactic acid by using it as a nutrient source and converting them to weaker acids, or by generating alkali from the metabolism of arginine or urea in saliva. These findings provided support for the ‘nonspecific plaque hypothesis’, in which caries is a consequence of the net metabolic activity of the biofilm41. More recently, studies using classic culture or molecular approaches have found associations between caries and other groups of acid-producing and acid-tolerating bacteria, including a range of Bifidobacterium spp., Actinomyces spp., Propionibacterium spp. and Scardovia wiggsiae.
Subsequently, alternative concepts have been proposed based on ecological principles that describe the events associated with caries42,43; these ecological plaque hypotheses are now generally accepted as the most plausible explanations of the microbial aetiology of caries (Fig. 4). The original ‘ecological plaque hypothesis’ recognized the consistency of bacterial function (that is, rapid acid production and tolerance of the acidic conditions generated) in the absence of specificity in bacterial name, and emphasized the essential requirement of a caries-conducive environment (that is, sugar-rich diet and/or low saliva flow). Microorganisms with traits that are relevant to caries can be present in biofilms on sound enamel, but at a level or activity that is too low to be clinically relevant42. Caries is a consequence of an unfavourable shift in the balance of the resident microbiota driven by changes in the dental environment. The regular exposure of plaque to fermentable dietary sugars results in repeated conditions of low pH in the biofilms that favour the growth and metabolism of acid-tolerating bacteria while inhibiting beneficial organisms that preferentially grow at neutral pH. Implicit in this hypothesis is the concept that disease can be controlled, not only by directly inhibiting the implicated bacteria but also by interfering with the factors that drive the deleterious shifts in the microbiota (that is, reducing the amount and frequency of sugar intake to prevent acidic conditions or promoting the use of snacks containing alternative sweeteners that cannot be metabolized to acid by oral bacteria)42. The ecological plaque hypothesis has recently been developed further to reflect the ability of some oral bacteria to adapt to acid stress during regular and prolonged conditions of low pH, termed the ‘extended caries ecological hypothesis’ (Ref. 43). Again, acidification of the plaque acts as the main factor selecting an acid-generating and acid-tolerating bacterial community, the development of which increases the risk of caries43.
Thus, dental caries is not an example of a classic infectious disease but is a consequence of an ecological shift in the balance of the normally beneficial oral microbiota, driven by a change in lifestyle and in the oral environment. An appreciation of these principles opens new avenues for caries prevention.
Environment and dental caries
Although biofilm formation is a natural process and is an essential step for caries formation, the presence of a biofilm on a tooth surface is not in and of itself an indication that disease is present. It is only after a complex interaction of host factors — including the tooth surface, acquired pellicle and saliva, and free sugars in the diet — that the presence of the dental biofilm can lead to disease expression over time.
The unique environmental conditions that exist at each tooth site explain the highly localized and complex nature of the caries process, whereby caries can occur at a specific location on the tooth surface and not on an adjacent tooth surface, even when both appear to be covered by biofilm27. These include tooth-related factors that impact acid solubility (for example, tooth composition (such as imperfectly formed structure as in hypoplasias) and structure, pre-eruptive and post-eruptive fluoride exposure, and post-eruptive age of the tooth), and those that influence biofilm thickness and pathogenicity by creating areas of plaque stagnation (for example, tooth morphology, arch form, occlusion and tooth position)36,37. Development defects (for example, enamel hypoplasia, a condition characterized by thin enamel) may lead to increased acid solubility and loss of surface structure, which creates sites of plaque stagnation and increases the risk of caries in primary teeth44. Caries susceptibility can also be affected by the proximity of teeth to salivary gland orifices, and salivary film thickness and velocity at specific tooth sites36,37. Dental appliances (such as orthodontic appliances and dentures) and faulty restorations can also increase caries susceptibility at specific tooth sites by creating areas of stagnation, encouraging biofilm formation27.
Diagnosis, screening and prevention
Diagnosis, risk assessment, screening and prevention are all vitally important considerations for the successful understanding and control of dental caries at both the individual and the population levels. In many countries, screening has a specific public health definition that is discrete from clinical practice, but this topic is beyond the scope of this Primer. The focus here is on what happens at the individual patient level, whereby high numbers of patients daily interact with oral health professionals around the world. It must be emphasized that, to prevent and control caries, both public health and individual level interventions need to be optimized and aligned.
The International Dental Federation (FDI)1,45 and dedicated meetings46 have reviewed the caries risk and classification systems that are available. Although excellent work in developing a range of assessment systems has been carried out in some countries (Box 2), there is a shortage of comprehensive, internationally applicable, evidence-informed, holistic clinical systems, let alone ones developed by formal consensus processes. Thus, although fully acknowledging that there are a range of other systems for undertaking some parts of the clinical tasks required to inform modern caries management, we use the International Caries Classification and Management System (ICCMSTM (Refs 47,
Public health management. The goal of dental caries prevention is to preserve sound tooth structure, prevent demineralization of enamel and promote natural healing processes46. Interventions can be implemented at the population level with health policy, legislation, regulation and public health approaches to promote healthy behaviours and affect broader social determinants of health51,52. Prevention approaches may target an entire population (for example, water fluoridation and sugar taxes) to assure equity, or they may target higher-risk groups to seek to increase cost-effectiveness. The subject of fluoride and caries is considered in Box 1, and there is further consideration of fluorides and caries prevention in Box 3. Some caries risk factors at the population level include low family income, restricted dental care access, low fluoride exposure, low oral health literacy and high caries prevalence. Research is ongoing to find the best ways to target individuals with a high risk of developing caries. Prevention programmes can be targeted at groups with medical or special health care needs, such as those with compromised immunity (for example, individuals with HIV infection or leukaemia), cognitive or developmental disabilities that can make oral hygiene difficult, genetic disorders that are associated with oral conditions (for example, cleft lip and palate, and ectodermal dysplasia), salivary dysfunction from Sjögren syndrome, diabetes mellitus, or frequent use of some medications that cause dry mouth (for example, antihistamines).
As dental caries is a multifactorial disease, complementary interventions may be more effective than single interventions. The WHO's oral health action plan emphasizes the need for oral health prevention programmes to be combined with other chronic disease prevention and educational programmes and policies that share common risk factors53. In the future, shared electronic health records, mobile smart devices and social media may assist in these efforts54. The Alliance for a Cavity-Free Future (an international public health advocacy charity) has chapters worldwide promoting a comprehensive agenda of activities and resources to prevent caries initiation and progression. Advocacy and education efforts include increasing public awareness and behavioural changes to improve oral hygiene and decrease sugar consumption, advancing research and clinical caries management55,56.
Many water supplies contain naturally occurring amounts of fluoride. Community water fluoridation — the adjustment of fluoride levels to community water supplies to obtain optimal levels for caries prevention — is a cost-effective, equitable population approach that best meets public health criteria and benefits all age groups57 (Boxes 1,3). The US Public Health Service recommends the fluoride concentration of 0.7 mg per l to maximize caries prevention while minimizing the risk of dental fluorosis58. This approach is likely to provide societal cost savings59. The WHO recommends a higher concentration (1.5 mg per l) but flags that the expected volume of water consumed and intake of fluoride from other sources should be considered when setting national standards60.
Salt fluoridation, often combined with iodized salt, is an effective, frequently used population-based method of caries prevention. It is used primarily in Europe, Central and South America where fluoride in the drinking water is low, community water fluoridation is not feasible and other forms of fluoride are used less frequently61. The fluoride concentration in salt is usually 250–300 parts per million (Ppm). It is the least expensive method of caries prevention62. Milk fluoridation programmes have been used in some countries, such as Hungary and the United Kingdom63,64.
Dental sealants are professionally applied resin material that are brushed onto the caries-prone pit and fissured grooves of the occlusal chewing surfaces of children's molars (the back teeth) to prevent disease in healthy teeth or arrest progression of initial non-cavitated carious lesions65. The application does not require local anaesthesia, and sealants can be applied in school-based programmes with portable equipment. Sealant programmes are an effective community approach66 that can be cost-effective when applied to children at higher risk of caries or from low-income families67. In the United States, schools with many children enrolled in the Free and Reduced Price Meal Program are targeted for school-based interventions68.
Although preventive strategies should be cost effective and result in societal cost saving, prevention programmes incur many up-front costs, whereas the savings gained by averted disease and treatment may take years to accrue69. The impact of dental pain and infection on quality of life also needs to be considered. Economic assessments of preventive strategies depend on many factors, including caries prevalence, personnel and material costs, intervention effectiveness and time frame. Examples of results from two different populations70,71 are compared in Box 4.
Individual patient level. Many of the same approaches and technologies used at the population level are also appropriate for use in the dental office or community clinic level. A range of evidence-based toolkits72, clinical guidelines73,74 and Cochrane systematic reviews75, taken together with the WHO guideline on sugar consumption76, provide a rich evidence base to support caries prevention in individuals presenting at the dental office. This is built around advice to limit the amount and frequency of sugar intake (which is also linked to obesity and type 2 diabetes mellitus prevention) as well as the frequent use of fluoride-containing toothpastes, supplemented according to caries risk status by fissure sealants and more intensive preventive interventions.
Risk assessment. The first element in assessing an individual for caries according to the ICCMSTM system is to determine patient-level caries risk by taking a comprehensive history asking a series of questions known to be associated with increased caries risk or caries protective factors. This includes assessment of the medical history and the relevant social history (for example, where the patient was born and raised, the present residence, education level and occupation). Finally, the patient is asked about diet conditions in terms of daily sugar intake and frequency, the number of between-meal snacks and the type of toothpaste used: all information important to assess the caries risk at the individual level77.
A wide range of risk assessment tools can be used and are compatible with the ICCMSTM system. One such risk assessment is Cariogram78, for which there is more evidence than for many alternative systems; studies have shown moderate accuracy on children and young adults79. Caries Management by Risk Assessment (CAMBRA) and other university-compiled risk factor questionnaires are alternatives. The Cariogram uses nine predicators in its full form: the DMFT, related diseases, diet content, diet frequency, amount of plaque, levels of S. mutans, fluoride use, saliva secretion and buffer capacity. A low score (that is, 0 or 1) indicates that a particular predictor contributes to low risk, whereas a high score (of 2 or 3) indicates high risk; thus, an overall risk can be estimated if the patient's profile remains stable78.
The risk assessment (independently of how it is derived) can be built later into the ICCMSTM caries risk-likelihood matrix49,50, which combines clinical caries activity with risk-level assessment. Eventually, it is possible to assess whether the patient is in low, moderate or high risk of developing more new lesions or progression of the existing caries lesions within the next few years. The risk of developing dental caries can be lowered by effective dietary advice, improved plaque control and increased use of fluoride, for example, by using 1,450 Ppm fluoridated toothpaste instead of 1,050 Ppm toothpaste80, assuming that the patient is compliant.
Clinical assessments. In order to find and assess any caries lesions on individual tooth surfaces, a clinical examination is performed (the detect and assess element of the ICCMSTM 4D methodology). The goal is to find any caries lesions present and assess their severity, activity and the risk factors at the tooth level. Assessments can include saliva secretion (and in some countries, buffer capacity and the presence of S. mutans are measured). Although these latter tests might increase patient motivation to implement preventive strategies, increasing knowledge as to the complexity of the biofilm is making such tests less clinically relevant. The level of plaque present is assessed clinically as it helps in assessing caries activity. Professional tooth cleaning then allows the identification of initial-stage81 and later-stage lesions, both of which are best detected on clean (plaque free), dry teeth. Location of gingival bleeding owing to gentle probing is also noted as a further indicator of lesion activity. Clinical examination alone is, for many patients, insufficient to make a complete assessment of caries. This is because, using visual examination alone, the dentist or dental hygienist may miss a substantial number of initial and even some more severe lesions82,83 (Fig. 6a).
The mouth is then dried with cotton rolls, and an individual tooth is examined by using the dental compressed air syringe. Lesion severity is then assessed according to the clinical appearance of the tooth surfaces47,
An assessment of caries activity of the detected and staged lesions can then be made. In the ICDAS classification system48,84,85, several predictors are used: the location of the lesion (whether it is a plaque stagnation area); the colour of the lesion (whitish versus brownish); tactile feeling (rough or smooth) when a blunted probe is run over the lesions, whether the lesion is matte or shiny, cavitated or non-cavitated; and, finally, whether the lesion is located along the gingival line and whether the gingiva bleeds after probing. A lesion is categorized as active if it has more of the following characteristics: has a plaque stagnation area, is whitish, has a rough tactile feeling, is matte, cavitated, and if there is gingival bleeding; the final diagnosis of the lesion is classified as initial, moderate or extensive active. If the lesion is categorized as inactive, the final diagnosis of the lesion is classified as initial, moderate or extensive arrested47.
Radiographic assessment. Radiographic examinations have been an important part of caries assessments ever since diagnostic radiography became available (Fig. 6b). However, there is a delicate balance between the diagnostic benefit and the small but real risks of using ionizing radiation electively. Thus, decisions as to when and how often to take dental radiographs for caries detection depends on factors such as the result of a thorough clinical examination, the patient's caries risk status, their age and the time since the last radiographs. The so-called bite-wing radiograph provides valuable information for caries in the tooth crowns, but these must be supplemented by additional views for some patients.
Although the dose of ionizing radiation has reduced with improvements in X-ray generation and the speed and sensitivity of films and sensors, there is still a responsibility to keep exposures as low as reasonably achievable and to use other diagnostic information where available to minimize radiographic exposures. Examples of clinical indications for a radiograph could be: suspicion of a caries lesion for a surface that is not easily inspected visually and assessment of the depth of a lesion, which is, at least partly, visible clinically.
It should be appreciated that, for some surfaces, particularly approximal surfaces where the site of caries attack cannot be visualized directly, radiographic information can be pivotal in thoroughly assessing the extent of caries lesions in terms of depth towards the dental pulp. However, this information must be combined with knowledge about caries activity status and overall caries risk when integrating all information into a personalized care plan, including decisions to treat non-operatively or operatively (the ‘decide’ element in the ‘4D’ system). Radiographic information can also be invaluable when investigating suspicious occlusal fissures in which, in a high-fluoride environment, an apparently small lesion may on occasion extend radiographically into the inner dentine. These examples highlight the importance of combining and integrating the clinical and the radiographic information to make accurate diagnoses from which to build appropriate care plans. Recall frequency for both clinical and radiographic examination should be decided on an individual basis, according to caries risk.
At the end of the examination, information on lesions visually detected and/or radiographically detected can be combined to make a diagnosis (Fig. 6).
Management of inactive and active lesions (with various degree of severity) is outlined below and in Fig. 7.
The initial lesion (even when active) is managed through non-operative care using remineralization therapy, involving behavioural changes and promotion of mineralization over demineralization, typically by using fluoride-containing procucts1. Remineralization is aimed at stopping progression of the lesion or ideally, reversing it. As part of the mineralization therapy, management should involve reviewing the dietary and oral hygiene behaviours (plaque control) of the patient, followed by education and encouraging behavioural changes86.
Fluoride can increase the rate and magnitude of remineralization of initial lesions87,88. Fluoride can be delivered topically either as a paste, gel or varnish by a dental professional or in the form of toothpaste, gel or mouthwash at home89 (Box 1). The acidulated fluoridated products have a low pH of 3.0–4.0. Reducing the pH of the fluoride vehicle has been demonstrated in the laboratory to prolong the ingress of mineral ions into the lesion by preventing the blockage of superficial enamel pores, which allow access to the deeper areas of the lesion86,90. Ideally, full recovery of an initial caries lesion would be achieved when calcium and phosphate are penetrating at high enough concentrations in the presence of slightly increased concentrations of fluoride91. Saliva is the most important source of ions around the teeth, and its flow can be enhanced through chewing sugar-free gums92. Fluoridated drinking water has proved to be effective by increasing the local concentration of this ion in saliva and plaque93. More recently, attempts have been made to further enhance the remineralization potential by incorporating water-soluble bioavailable calcium and fluoride into topical products, such as sugar-free gums, dental creams and varnishes. These formulations include casein phosphopeptide-stabilized amorphous calcium phosphate and phosphoryl oligosaccharides of calcium and their fluoride-containing variations94,
Although a non-surgical approach to the management of initial lesions seems to be the most beneficial, minimal intervention therapy can be used as well. Pit and fissure sealants have been applied using resin based material and glass ionomer cement. A newly developed resin-based product with surface pre-reacted glass-ionomer filler is expected to release fluoride sustainably because of its ability to recharge fluoride ion97. These fillers release other ions, enhancing tooth mineral formation98.
When an initial lesion is found, long-term monitoring combined with managing the caries risk factors is an option to consider. Surgical intervention can be considered only if the initial lesion advances99,
The goal of non-invasive management of moderate lesions (Fig. 7) is to arrest further progression and regain lost minerals. Two options are available.
Mechanical blocking. Clinical evidence shows that non-cavitated lesions on occlusal surfaces can be arrested with resin-based fissure sealants102. Extrinsic substrates are blocked, as acids produced by the biofilm cannot reach the enamel, and the number of bacteria in the carious dentine is reduced. The procedure strictly requires dry conditions, and the sealants must be regularly checked and maintained. A related technology to arrest proximal lesions is resin infiltration of the dentine layer103. Lesions limited to the outer third of the dentine are treated with a resin that penetrates and repairs subsurface pores. Although long-term results are lacking, it seems clear that the micro-invasive treatments have lower long-term costs than invasive therapy104,105.
Fluorides. Self-applied and professionally applied fluorides can remineralize and arrest caries lesions (Box 1). Topically applied silver diamine fluoride is a cost-effective alternative to arrest ECC and root caries lesions in elderly individuals, especially when other options are absent106,107.
Soon, novel nanotechnologies based on peptides and hydroxyapatite crystals, together with biofilm engineering, are expected to advance the non-invasive restorative options for moderate caries lesions108,109.
Extensive lesions are still most commonly subjected to classic standard care: the demineralized tissues are completely removed and replaced with a filling material. However, the development of adhesive techniques without need of mechanical retention (that is, composite resins or tooth-coloured mixtures of plastic and glass) has allowed dentists to adopt a more tooth-preserving approach. However, low-quality evidence suggests that resin composites lead to higher failure rates and risk of secondary caries than amalgam (a metal-coloured mixture of mercury and other metals) restorations110.
A recent development is the stepwise or partial caries removal, in which only the superficial layers of the lesions are removed. Systematic reviews have concluded that this approach reduces the incidence of pulp exposure and favours caries arrest and tertiary dentine formation (that is, laying down new protective dentine in response to an advancing caries lesion in both primary and permanent teeth)100,111. Although these techniques show clinical advantage over complete caries removal, it is too early to recommend certain clinical strategies. It must also be underlined that the decayed teeth must be vital and asymptomatic. Furthermore, the success depends on an appropriate restoration that completely seals the tooth and keeps remaining bacteria in the deeper dentine layers dormant. A disputed mode of managing advanced asymptomatic lesions in primary molars is the Hall technique112, whereby the tooth is not prepared in the conventional way but is covered by a preformed stainless steel crown; this technique shows a superior clinical performance than traditional restorative care113.
Children with a very high caries risk
To prevent caries and have successful management of oral diseases, perinatal and infant oral health care are essential aspects of early intervention, which facilitate behavioural changes and promote good oral health114. An essential step is to start educating caregivers and their health care providers on the importance of dental care during pregnancy and infancy, with the development of child-specific oral health measures and methods to prevent oral diseases115. This is especially important for children who are at high risk of ECC development. Although there is no mechanistic difference in the pathology and principles of care between ECC and other forms of caries in children or adults, the issues related to ECC have more association with specific behavioural risk factors — the most important of which is the use of night-time bottles containing sugary beverages and juices. Although anatomical differences between primary and permanent teeth exist (such as the enamel being much thinner in primary teeth and, therefore, caries can progress faster into the dentine), the mechanisms, pathophysiology and treatment approaches are identical between both types. However, clinical management for the younger patient and the involvement of caregivers brings its own challenges.
CAMBRA is an evidence-based approach adapted to the specific needs of the subset of the child population with ECC who experience a very aggressive caries challenge115,116. CAMBRA for ECC assists health care providers in a structured manner to first assess caries risk and the risk of caries progression at an early age in a patient-centred approach, on the basis of age and dental status and risk factors; second, to tailor a specific individualized care or preventive management plan, before deciding on a surgical modality; third, to formalize a follow-up plan based on risk and age of the child; and fourth, to ensure specific guidance for the caretakers with targeted self-management goals based on the age, risk and need of each individual patient at any given time. This version of CAMBRA provides the information to assess the risk of caries development and disease progression in young children as low, moderate or high risk.
Assessments can also be done through the use of the risk-assessment technique, whereby the focus is on three specific domains: risk and/or biological factors (such as continual bottle use, sleeping with a bottle, frequency and types of snacks or whether the child is taking any medications and some other risk factors); protective factors (such as the use of fluoridated tap water, use of fluoridated toothpaste or the continual use of xylitol (recommended by some but with mixed evidence)); and clinical findings from which health care providers can assess the presence of early demineralized enamel surfaces or cavities at a very early age, the presence of plaque biofilm and lack of salivary flow, among others117,118.
Through these guidelines for children provided by CAMBRA, early intervention can be conducted in primary care settings. Furthermore, providers are recommended to use minimally invasive treatments such as fluoride varnish119. At home, the caregiver should be guided and supported to adopt good oral health behaviours for their children and themselves and to use a small amount, no more than a grain of rice, of fluoridated toothpaste as soon as the first tooth appears, to protect the teeth from developing caries120. Ultimately, these practices will benefit the patient through the prevention and self-management of ECC, and they have the potential to deliver better practice, improve clinical outcomes and reduce the overall burden of disease in young children117.
Follow-up or recall
The final element of continuing caries care is to estimate when the patient needs to come to the clinic again, and this depends on the age and risk status of the patient. It is no longer deemed appropriate that all patients should be recalled every 6 months. If the risk assessment indicates low risk, the next visit can be postponed for >1 year for adults; a moderate-to-high risk status implies that the recall should be shorter. After a suitable risk-based recall interval, the caries management cycle starts again (Fig. 5).
Quality of life
Having considered the scientific, clinical and public health aspects of caries, it is important to appreciate the impact that the disease has on quality of life across the life course. The demographic transition towards ageing societies, and the oral health transition with considerably more people keeping their natural teeth into old age, has resulted in a relative shift in the burden of untreated caries towards adults121. Caries is still a highly prevalent condition among adults (18–65 years of age) and older adults (>65 years of age)121,
Toothache is still prevalent among children and adolescents, and is strongly associated with dental caries, particularly among groups of lower socioeconomic position, with an estimated 5–6% increase in the probability of toothache for each additional primary tooth with caries experience125. In the most recent national study in the United Kingdom, 18% of those 12 years of age and 15% of those 15 years of age reported toothache126. Even though dental caries can be asymptomatic, particularly at its initial stages, caries is associated with diminished quality of life for people affected and their families127. Among children, caries is associated with negative impacts on a range of daily life activities128,129, and this was also the case among very young children, in whom caries was associated with worse oral health-related quality of life in terms of perceptions of both children and their parents130. Similarly, toothache and tooth decay are the conditions most commonly associated with worse oral health-related quality of life in adults. Among adults in England, Wales and Northern Ireland, 16% reported frequent and 17% reported severe impacts on their daily life owing to their oral conditions, but the respective prevalence for both frequent and severe oral impacts was 24% among those with decay and 38% among those with experience of severe caries as expressed through the PUFA (pulpitis, ulceration, fistula or abscess) Index122. Globally, untreated caries accounted for almost 5 million disability-adjusted life years (DALYs) in 2010, with a further 4.5 million DALYs attributed to excessive tooth loss. Caries contributed to DALYs across the different ages, but more so for children and young and middle-aged adults. Indeed, caries are the predominant oral health cause of DALYs among people ≤35 years of age, whereas extensive tooth loss was prevalent and an important contributor to DALYs among middle-aged and older adults18.
The impact of dental caries is not limited to oral symptoms and the detrimental influence on the quality of life. Caries in primary dentition is associated with malnutrition131; children with severe ECC have relatively poor nutritional health for a range of nutrients compared with children without caries132,133. Other studies have shown a link between caries experience and poor child growth and low weight gain134,
The impacts of caries disproportionately affect the more deprived groups in the society122,125,126,130,145, in line with the evidence supporting links between clear socioeconomic inequalities with higher risk of caries lesions or experience among those in lower socioeconomic positions146. This highlights the importance of focusing not only on behavioural and biological risk factors but also on the broader social and environmental determinants of the disease147.
Dental caries remains one of the most prevalent global chronic diseases9,148. For at least a century, dental caries has been managed surgically. Public health measures, such as water fluoridation and topical fluorides, have had considerable impact on the burden of dental caries in developed countries. However, the failure to eradicate or halt the burden of this disease in many parts of the world, as well as vulnerable population groups in developed countries, means that dental caries remains a major public health problem.
In the emerging era of health outcomes and value-based health care systems149, the success and payment for the provision of health care will be determined not only by the type and number of procedures provided to patients but also by the final health outcomes achieved. Management of dental caries should move forward to a model in which health outcomes form the basis for compensation and are the focus of dental care. Accordingly, the major outcome of caries management clinically, personally or at a community level is the preservation of tooth structure and maintenance of teeth in a healthy state (outcome one). A second outcome would be to control initial stages and arrest their progression or reverse the caries process towards health (outcome two). These two outcomes precede the restoration of lost tooth structure and function of decayed teeth (outcome three), which is the current major outcome that is being reimbursed by third party payers or patients all over the world. To achieve these three outcomes, process outcomes must be evaluated throughout the care cycle and be reimbursed accordingly.
Dentists and other dental professionals must stage the caries progress, assess risk factors (on the dental, medical, biological, behavioural and social level), develop comprehensive management plans to prevent new caries based on risk status of patients, control initial lesions, restore cavitated lesions, rehabilitate the dentition and develop a follow-up plan54. Assessing and reimbursing dentists to evaluate all these process outcomes is as important as reimbursing them for achieving the health outcomes through procedures because it is through these outcomes that dental health is achieved. There is no doubt that valuing the newly proposed outcomes represents a revolutionary change in dental care globally, which, as stated before, has so far been focused on delivery of procedures. A procedure-focused system of reimbursement is not obsolete, but rather it must become part of a larger model of health-promoting system of care. Implementation of a new paradigm in dental practice and education, as expected, will not be easy and will take time.
Fortunately, there are a few practical examples of success in changing paradigms of care in dentistry. For example, in the 1970s and 1980s, a series of clinical trials compared non-surgical and surgical periodontal therapies and found that, for most levels of severity of periodontal diseases, both modalities produced similar outcomes150. Although there was vociferous reaction from supporters of the surgical modalities of care, the emergence of evidence led to a radical shift over two decades towards non-surgical care. It may be that similar changes to a less interventional approach to caries management can change practice151. Another pragmatic model for implementation is the development and dissemination of guidelines and policies that direct dentists into adopting the desired behaviours. Dentists are used to adopting, although with some hesitation, standards dictated by dental insurance companies, government agencies, licensing boards or health authorities. The prime example of the change in behaviour due to policies is the adoption of infection control measures and standards for occupational health and radiation safety.
Other than those two pragmatic experiences, there is a dearth of evidence on implementation of changes in dental practice152. Continuing education and articles in dental journals, although necessary, are by themselves insufficient in their effectiveness to influence change in practices of practitioners. The case to support this conclusion is clear in the adoption of pit-and-fissure sealants. Although sealants are based on the same materials as resin composite fillings, their adoption has been slower than the rapid growth in use of tooth-coloured filling materials153. The reason for this may be because when sealants are promoted, based on the evidence154, as a procedure of choice for non-surgical management of initial caries lesions, they replace a standard operative procedure that dentists have adopted as the norm. By contrast, tooth-coloured restorative materials replaced amalgam ‘silver’ filling materials, which dentists and patients started to abandon as a norm because of concerns about the exposure to mercury and aesthetics.
It is expected that the proposed integrated caries management system and value-based dental care require for many, but not all, changes in the current norms of practice. The way to move forward in implementation will require a multi-pronged strategy, collective engagement of all dentists, educators and policy makers, and research studies that assess the outcomes of the new proposed paradigm of caries management, and refine it as needed. It should be appreciated that timescales and tipping points in implementation are notoriously unpredictable.
The burden of dental caries in the world is considerable in terms of cost, loss of time from work and school, in some cases severe facial and systemic infections, and rarely death. Hence, it is imperative that the new approach described in this Primer is implemented now. A collaboration among dental schools, clinics, and professional and government agencies must be formed to create a learning organization that shares experiences and assists in conducting research of the previously described outcomes. The initial steps of this work are under way with a consensus production of a guide in caries management for practitioners and educators49. Fortunately, there are today new tools that can aid and empower changing norms of practice and collecting outcome data. The use of computerized reminders, electronic audit and feedback, and stop-and-go decision algorithms in electronic health records, provide new venues to help in changing practitioner behaviours. Audit and feedback have been found to be modestly effective in changing health care behaviours155. Incorporation of instantaneous audit and feedback in electronic health records could provide reminders to practitioners to follow standards of care during the care process.
The task ahead will be to develop new tools and reimbursement incentives as well as form a collaboration to coordinate the implementation of the new caries management system across fields. The collaboration will also design and assist partners in conducting research on outcomes of the new caries management system. It is time now to start the process of moving towards preserving tooth structure, rather than repairing it with artificial materials.
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How to cite this article
Pitts, N. B. et al. Dental caries. Nat. Rev. Dis. Primers 3, 17030 (2017).
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