Introduction

Dental caries is a biofilm-mediated, sugar-driven, multifactorial, and dynamic disease characterized by alternating phases of demineralization and remineralization of the dental hard tissues.¹ The pH and carbohydrate availability are key environmental factors affecting the physiology, ecology, and pathogenicity of the oral biofilms colonizing the teeth.² The acidic environment that triggers the demineralization of tooth enamel results from the metabolic activity of cariogenic bacteria, such as Streptococcus mutans, which ferment dietary sugars and produce acid as a byproduct.³ Changes in environmental pH occur following the consumption of dietary sugar. Specifically, organic acids produced by the fermentation of dietary carbohydrates by cariogenic bacteria elicit demineralization of tooth enamel. These periods of acid challenge to the tooth are followed by periods of alkalization, which neutralizes plaque pH and promotes remineralization of tooth enamel.⁴

In discussions of the caries process, particular attention is given to the enamel - the hard, outermost layer - because it is the primary contact with cariogenic bacteria and the starting point of the demineralization process that can lead to caries development. Enamel is composed predominantly of hydroxyapatite crystals, which are vulnerable to acid dissolution, especially when there is a loss of calcium and phosphate ions.⁵ Many beneficial oral bacteria can tolerate short periods of low pH, but their growth is inhibited by prolonged or frequent exposures to acidic conditions.⁶ Enamel is also the only tissue of the tooth that does not have the ability to grow or repair itself after maturation, making it even more crucial that its demineralization is prevented. Once enamel is demineralized, it cannot regenerate on its own without intervention, which is why maintaining its integrity is essential in caries prevention. Caries can also develop in dentin, the hard layer under the enamel, so understanding the chemical composition of this layer, and how it is affected by demineralization, is also important.

The process of remineralization - the replacement of lost minerals in hard dental tissues - can halt, slow down, and, in some cases, reverse the caries process.¹ Saliva and fluoride are two key players in remineralization. Saliva contains calcium and phosphate ions, which help restore lost minerals and strengthen the tooth structure. Saliva also plays a key role in maintaining oral pH at around neutrality, which is optimal for the growth of most health-associated oral microbiota. Fluoride can be incorporated into the tooth structure to form fluorapatite , which is more resistant to acid dissolution than hydroxyapatite.⁵ For caries prevention, factors in the oral cavity must be highly favorable for remineralization to occur, so that this process can be effective. If the environment is more favorable for demineralization, such as in the presence of low pH or high bacterial acid production, the remineralization process may have little or no influence, or not occur at all; and caries will develop.

Clinical Significance Snapshots

How does understanding the demineralization-remineralization cycle help me prevent or arrest the caries process in my patients?

The ‘demin-remin’ cycle is like the ebb and flow of money in a checking account. If too many withdrawals are made and too few credits received, the account becomes overdrawn. When deposits (remineralization) match or exceed the withdrawals (demineralization), a healthy balance is maintained. The same applies to the balance of calcium, phosphate, and fluoride ions entering and exiting the tooth. Some loss of these minerals inevitably occurs during mealtimes, as cariogenic bacteria in the biofilm on the surface of the tooth metabolize the sugars in the diet via glycolysis. This produces lactic acid, which creates a low pH environment (often dropping below pH 5.5) that is capable of driving demineralization of enamel. Between meals, saliva plays a crucial role in neutralizing acids and buffering the pH back to safe levels (above pH 5.5), allowing calcium, phosphate, and fluoride ions to return to the tooth surface for remineralization. However, if there is not enough time for sufficient remineralization (for example, if dietary sugars are consumed frequently throughout the day), then there is an overall loss of calcium and phosphate from the tooth, leading to the development of subsurface caries lesions. If the demineralization process continues unchecked and the subsurface lesion continues to lose minerals, the enamel surface will lose structural support, eventually resulting in the formation of a cavity. To prevent the occurrence of caries in your patients, it is important to include information about foods that contribute to demineralization, such as sugary or acidic foods and beverages, in oral health counseling. It is also essential to emphasize that saliva requires time between food intakes to restore any loss of minerals caused by sugar-containing foods and beverages. Foods rich in calcium, such as yogurt, cheese, or milk, can help remineralize enamel. These foods are particularly beneficial at the end of meals to help neutralize acids and promote remineralization.

Why is the use of fluoride agents so prominent in the prevention of dental caries?

Hydroxyapatite crystals in enamel are naturally impure due to the presence of carbonate ions. These carbonate ions weaken the crystal structure, making it more vulnerable to dissolution by acids produced during the fermentation of carbohydrates by cariogenic bacteria. Fluoride ions can replace some of the carbonate and hydroxyl ions in hydroxyapatite, forming fluorapatite. Fluorapatite is structurally stronger and more resistant to acid dissolution than carbon-hydroxyapatites. This conversion significantly enhances enamel’s resistance to demineralization, effectively tipping the demineralization/remineralization balance in favor of remineralization. Fluoride should be available in the oral cavity daily in low concentrations, ideally through the use of toothpaste with proven bioavailability of fluoride (ADA Seal of Acceptance), to consistently promote enamel health and prevent caries. For patients who are at higher risk of caries, such as those with frequent sugar consumption or reduced saliva flow, additional forms of fluoride application should be considered. These may include fluoride rinses (daily or weekly depending on concentration), and professional application of fluoride varnish, gels or foams, which offer higher fluoride concentrations for enhanced protection.

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