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Profile in Oral Health: Understanding the Caries Process by Trisha E. O’Hehir, RDH, MS

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Enamel Caries vs. Dentin Caries
by Trisha E. O’Hehir, RDH, MS

Introduction

Enamel is the hardest substance in the human body. It is in a constant state of flux, going back and forth from demineralization to remineralization. Every time there is a drop in the oral pH below 5.5, enamel demineralizes. Drinking orange juice, wine or soda will demineralize the enamel. Remineralization then occurs with the help of salivary minerals and buffering agents. This natural ebb and flow can be disrupted by continuous exposure to acid, usually in areas protected from the remineralizing benefits of saliva. When bacterial biofilm covers pits and fissures and interproximal sites, the acid is held against the tooth surface with no remineralization possible. The demineralization affects enamel, eventually visible as white spot lesions. The next step is cavitation and eventually the demineralization moves into the dentin.

At this point, the process changes. Bacteria-produced acids begin the demineralization process in the dentin, and then endogenous, zinc-dependent proteases destroy the dentin. The bacteria are responsible for the initial demineralization and destruction of enamel, but then substances within the body that are often beneficial become destructive to the dentin. This two-phase destruction of tooth structure requires a variety of preventive approaches.

Traditionally, the focus was to protect the enamel with fluoride and by disrupting bacterial biofilm formation and reducing the consumption of fermentable carbohydrates. Based on new scientific findings, additional strategies are needed to inhibit the destructive actions of proteases, specifically the family of Matrix metalloproteinases (MMPs). Better understanding the carious process from enamel through dentin will provide new options for preventing and reversing carious lesions.

Caries in Enamel

Plaque biofilm is composed of a multitude of bacteria including Streptococcus, Lactobacillus and Actinomyces species. Oral bacteria convert carbohydrate foods and drinks in the mouth for their own energy through a process of fermentation. Lactic acid is a byproduct of this fermentation process and with a pH below 4 is capable of demineralizing enamel. This acidification of the biofilm causes demineralization of enamel. The acidification of the biofilm provides an environment conducive to the proliferation of acidogenic and aciduric bacteria, those that prefer a low pH environment and those that produce lactic acid. As long as the environment continues with a low pH, enamel demineralization will continue. No disruption of the biofilm and continued acid production will lead to more demineralization and eventual cavitation.

A constant source of fermentable carbohydrates feeds these acid-producing bacteria. Although often referred to as “sugar bugs,” it’s not just sucrose or table sugar that leads to acid production. Hydrolyzed starches can be fermented to produce lactic acid as well. These starches are long chain sugars that contribute to acid production. Potato chips, pasta, bread and other starches will all provide the nutrients necessary to continue the caries process. Although sucrose is the primary factor, hydrolyzed starches are also considered fermentable carbohydrates.

Caries in Dentin

The carious process within dentin differs from that in enamel. Dentin is less mineralized, containing 20 percent organic material compared to only one percent in enamel. The bacteria-produced acids that dissolve enamel will also dissolve the dentin mineral, uncovering the organic dentin extracellular matrix (ECM). Proteases are then responsible for degradation of ECM, allowing the movement of bacteria toward the pulp. The tubular nature of dentin enhances this movement of bacteria. It was long thought that the proteases degrading the ECM were produced by the bacteria, but recent findings suggest that bacterial proteases cannot withstand the drop in pH that often reaches 4.3.

New theory suggests that host-derived, zinc-dependent proteases, specifically Matrix metalloproteinases (MMPs) found within dentin and saliva, are responsible for the degradation of dentin. MMPs are involved in both normal and destructive actions throughout the body. MMPs consist of a family of endogenous proteolytic enzymes. Some are associated with dentogenesis and others are capable of degrading dentin. Active MMPs have been found in demineralized dentin, suggesting they can disorganize and degrade the dentin matrix. MMPs require metal ions, specifically zinc, for activation. Strangely, the MMPs must be activated, often by acids, but then require a neutral pH to destroy the matrix components. The bacteria-produced acids can activate the MMPs and it’s thought that salivary buffers then allow dentin destruction by the MMPs.

Examination of extracted, carious teeth shows a gradual change in the gelatinous texture of the dentin. The continuum includes a superficial soft carious lesion, an inner soft carious lesion, affected dentin and sound dentin.

The Caries/Diet Connection

Today’s diet no longer includes three meals and a snack after school, as was the trend years ago. Today, fermentable carbohydrates are consumed continuously throughout the day and into the evening. Snacks and fizzy drinks are readily available all day long. This contributes to the “ecological plaque hypothesis” introduced by Drs. Takahashi and Nyvad that the pH of the plaque biofilm determines disease activity. High intake of fermentable carbohydrates will favor acid production and proliferation of acid-producing oral bacteria. Changing the diet by reducing the intake of fermentable carbohydrates can elevate the pH, shifting the ecology of the biofilm to one more conducive to health.

Introducing oral probiotics may shift the balance of bacteria in the mouth to those preferring a higher pH. Competition between established acid-producing bacteria and specific species contained in oral probiotics leads to metabolism of lactic acid into hydrogen peroxide, which will inhibit S mutan growth. Xylitol also interferes with the sucrose glycolosis. Xylitol is a five-carbon molecule, not six like sucrose. The smaller molecular size allows xylitol to pass through the outer cell wall of the bacteria easily, but it is not the right molecular structure to be used by the bacteria to produce energy. The bacteria must then use its own energy to pump the xylitol molecule out via a membrane pump. This process expends energy without providing an energy source for the bacteria. Xylitol also blocks the communication between bacteria, interfering with quorum sensing, a key function in the formation and maintenance of biofilm structure. Without mechanical disruption of bacterial biofilm, three to five exposures to xylitol daily will reduce bacterial biofilm levels by approximately 50 percent.

Preventive Strategies Now and in the Future

Until now, caries prevention has focused on enamel caries, with no specific approaches to prevent dentinal caries. Control of diet, adequate biofilm removal and fluoride exposure are components of the current approach to caries prevention.

MMP inhibitors may be the next level of prevention focused on prevention of dentin demineralization. Research is now being done to determine if the use of chemical or natural MMP inhibitors can control caries progression within dentin. The tetracycline family of antibiotics can inhibit MMPs, separate from their antimicrobial properties. Zoledronate, a third generation bisphosphonate, is also a potent MMP inhibitor. However, these drugs are used systemically and a better choice will be a topical product. Chlorhexidine as well as Ethylenediaminetetraacetic acid (EDTA) will impair MMP activity and can be used topically.

Other potent MMP inhibitors come from natural sources, including green tea polyphenols and grape seed extract. Grape seed extract suppresses lipopolysaccharide-induced MMP secretion by macrophages. Grape seed extract was shown in laboratory studies to both inhibit demineralization and promote remineralization of artificial root caries lesions. Both chemical and natural ingredients can be incorporated into oral rinse and toothpaste products in the future.

Conclusion

The caries process involves destruction of both enamel and dentin, with a combination of damaging actions. Prevention and remineralization are two critical approaches to address with new scientific knowledge and modern technologies. Prevention and remineralization of early lesions are possible using both traditional and contemporary approaches.

Dentaltown Magazine
April 2024
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