Exposure of root surfacesIn the young, healthy adult, root surfaces, like the cementoenamel junctions, are not exposed to the oral cavity. At the population level, the prevalence of exposed root surfaces is strictly age related and is attributed to the long-term effects of trauma from toothbrushing (buccal surfaces) and gingival recession associated with periodontal disease. With the decline in prevalence and severity of enamel caries, and hence the preservation of an intact dentition into old age, root caries is becoming an increasing problem in clinical practice.

Enamel chemistryEnamel mottling apart, the fact that fluoride affects dental caries has been confirmed by many well-controlled studies of topical fluoride agents and studies of the posteruptive caries-preventive mechanisms of fluoride (for review, see Fejerskov et al, 1996a, b).For many years, it was believed that incorporation of fluoride into enamel increased the resistance of the tooth to dissolution and that the surface enamel fluoride concentration could be a marker of tooth resistance or susceptibility to caries.

Enamel structureEnamel development is conventionally described in five histologically recognizable stages: secretion (matrix deposition and transition), cell organization, preabsorption, early maturation, and late maturation. In later work, only four stages are described, based on chemical composition. Developmental disturbances may occur at any stage.  There is, however, no clear clinical evidence that aberrations in enamel structure affect resistance to dental caries, unless the defects are major and result in rough surfaces that enhance plaque retention.

Role of Tooth Size, Morphology, and CompositionIntroductionOne approach to the prediction of future caries incidence is to study the tooth itself, allowing for the fact that the environment of the tooth will be equally important. The various aspects of tooth resistance then take on greater importance. With this approach, the individual or group of individuals showing resistance to caries can be identified. Various aspects of the resistance of a tooth to dental caries can be described.

Future caries vaccineTo date there is no efficient vaccine against dental caries, particularly for early childhood, before colonization by the cariogenic microflora. The ideal determinant for use in caries vaccine would be one that induces antibodies that exert one or both of the following effects on S mutans and S sobrinus: (1) limit the colonization of the organisms in dental plaque; and (2) affect S mutans and S sobrinus in such a way that processes of importance for the development of caries (such as growth and production of acids and polysaccharides) are inhibited or reduced to a level not resulting in caries.

Role of Chronic Systemic Diseases and Impaired Host FactorsChronic systemic disease Of the systemic diseases, by far the greatest caries risk is associated with rheumatoid conditions, particularly Sjogren's syndrome, because of its severe depressive effect on the salivary secretion rate as well as the quality of the saliva. The most severe xerostomia is seen in patients with Sjogren's syndrome. Other systemic and chronic diseases that cause salivary gland hypofunction and xerostomia and are thereby regarded as risk factors and prognostic risk factors are listed in Box 9.

Preventive programs for patients with hyposalivation and xerostomiaFor such high-caries-risk patients, most preventive measures, self-administered as well as professional, must be optimized. The following regimens are recommended. Plaque control and self-administered fluoride Patients with impaired salivation are extremely fast plaque formers (Plaque Formation Rate Index scores 4 and 5). Therefore, not only the frequency but also the quality of combined mechanical and chemical plaque control by self-care has to be optimized, and all tooth surfaces must be targeted.

Symptomatic therapyIn the absence of natural salivation, it is essential to try to protect the oral hard and soft tissues by salivary substitution. Saliva substitutes, also called artificial salivas, are frequently recommended for patients complaining of dry mouth (xerostomia).  Although many studies suggest that saliva substitutes are useful in the management of xerostomia, clinical experience has shown that these products are not well accepted by patients. Most patients do not continue to use the substitutes regularly, relying instead on water or other fluids to relieve their symptoms.

Salivary stimulation and substitution in patients with hyposalivation and xerostomia - Stimulation of salivaRecognition of the key role of saliva in maintaining normal oral function has stimulated research on its protective properties against caries and on the treatment of xerostomia and salivary hypofunction. Salivary clearance, buffering power, and degree of saturation with respect to tooth mineral are the major protective properties (for review, see Sreebny et al, 1992; Tenuvuo, 1997), their effect increasing with salivary stimulation: The saliva stimulated by consumption of fermentable carbohydrates reduces the fall in plaque pH that could lead to demineralization andincreases the potential for remineralization.

Formation and functions of pellicleSaliva is seldom in direct contact with the tooth surface but is separated from it by the acquired pellicle, defined as an acellular layer of salivary proteins and other macromolecules, approximately 10 um thick, adsorbed onto the enamel surface. It forms a base for subsequent adhesion of microorganisms, which under certain conditions may develop into dental plaque. The pellicle layer, although thin, has an important role in protecting the enamel from abrasion and attrition, but it also serves as a diffusion barrier.

Antimicrobial and other protective propertiesThe saliva contains many different proteins and some other small organic proteins that  together protect the oral cavity (the soft tissues as well as the teeth) from frictional wear, dryness, erosion, pathogenic bacteria, and so on (see Box 12). Lubrication and other protective properties. Almost all salivary proteins are glycoproteins; that is, they contain variable amounts of carbohydrates linked to the protein core. Glycoproteins are often classified according to their cellular origin and subclassified on the basis of their biochemical properties.

Demineralization and remineralization of tooth surfacesThe physicochemical integrity of dental enamel in the oral environment is entirely  dependent on the composition and chemical behavior of the surrounding fluids: saliva and plaque fluids. The main factors governing the stability of enamel apatite are pH and the free active concentrations of calcium, phosphate, and fluoride in solution, all of which can be derived from the saliva (see Box 12).  The development of a clinical carious lesion involves a complicated interplay between a number of factors in the oral environment and the dental hard tissues.

Neutralization and buffering of acidsAlthough while the effect of saliva in facilitating sugar clearance can partly explain why saliva reduces formation of plaque acids and therefore caries, the neutralizing and buffering actions of saliva are more dramatic. These are due predominantly to salivary bicarbonate, originating mainly from the parotid gland. In unstimulated saliva, the bicarbonate level is low; at the greater secretion rates of stimulated saliva, the concentration is higher, the pH rises, and the buffering power of saliva increases dramatically.

Role of saliva as a modifying factor in dental cariesIndisputably, an adequate secretion rate and saliva of good quality are essential for oral health. Saliva is well known to have specific protective effects against dental caries. The most direct evidence of this is the rampant caries that can occur following the loss of salivary function as a result of irradiation for head and neck tumors. Within a few weeks, tooth surfaces not normally susceptible to caries may be affected, leading to complete coronal destruction.

Composition of salivaAlthough composed mainly of water, saliva is a complex secretion. As discussed earlier, so-called whole saliva consists primarily of the secretions from the major and minor salivary glands. Whole saliva also contains a number of constituents of nonsalivary origin: crevicular fluid, serum, and blood cells; bacteria and bacterial products; desquamated epithelial cells and cellular components; viruses and fungi; food debris; fluoride; and some bronchial secretions.

Evaluation of hyposalivationBased on the principle that "status is determined by clinical examination but  explained by the case history," the following points should be considered for proper diagnosis of hyposalivation:1. Stimulated salivary secretion rate2. Resting SSR3. Anamnestic data: possible side effects of medication; systemic diseases known to cause salivary gland hypofunction; difficulty in swallowing dry food; difficulty in speaking; soreness of the oral mucosa; frequent episodes of sore throat; difficulty in tolerating removable dentures4.

Causes of hyposalivation and xerostomiaThe salivary glands derive their fluid from the circulating blood. This fluid, with its  electrolytes and small organic molecules, is modified by the glands and, together with the macromolecules synthesized by the gland cells, secreted into the oral cavity (see Figs 80 and 81). Secretion occurs in response to neural stimulation. Disturbances of the blood supply to the gland, of its secretory apparatus, or of the stimuli that elicit secretion may lead to a decrease in the production of saliva.

Symptoms of salivary gland hypofunction resulting in hyposalivationApart from an increased susceptibility to caries, other oral and systemic disturbances may also be associated with hyposalivation (Box 7). Hyposalivation, or reduced SSR, is not synonymous with xerostomia, which is a symptom reflecting the end result of the process of inflow of pure saliva, evaporation, adsorption to the oral mucosa, and outflow of saliva. Of the saliva that enters the mouth, as much as 0.20 to 0.

Internal Modifying Factors Involved in Dental CariesIntroductionAs discussed in chapter 2, many factors modify the prevalence, onset, and progression of dental caries. The major internal (endogenous) modifying risk indicators, risk factors, and prognostic risk factors related to dental caries are reduced salivary secretion rate (SSR), poor salivary quality, impaired host factors, chronic diseases, unfavorable macroanatomy and microanatomy of the teeth, and the stage of eruption, all of which favor plaque retention, poor quality and maturation of enamel, and exposed root cementum or dentin.

Socioeconomic and behavioral factorsEarly establishment of good oral hygiene and dietary habits and regular use of fluoride toothpaste are of utmost importance. Several studies in infants and toddlers have clearly shown that such habits, as well as dental status, are strongly correlated to the parents' social class (particularly educational level), dental status, regularity of dental care (particularly preventive programs), and ethnic background (immigrants).  Organized oral health education programs at maternal and child welfare centers are therefore important strategies for reducing such inequalities.