Role of Fluorides

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To examine the role of fluorides in the caries process, we need to understand the concepts of demineralization and remineralization and how they function in both coronal as well as root surface caries.

In the past we thought the caries process was a simple but continuous dissolution of the enamel by acids of bacterial origin with dissolution beginning at the enamel surface and progressively "eroding away" the enamel surface. Today we know the earliest clinical evidence of the caries process to be the white spot lesion, characterized by a chalky white appearance. If explored, it seems to be structurally intact.

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Caries Process

The surface layer is intact but the zone of demineralized enamel can be noted beneath the intact surface layer. Calcium and phosphate are lost from the tooth (enamel and dentin) during the demineralization process and may be either precipitated together somewhere else in the tooth or lost to the mouth via the plaque and saliva.

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Caries Process

In the remineralization process, calcium and phosphate diffuse into the tooth from saliva and/or plaque fluid and precipitate as new material inside the early carious lesion. Remineralization is an enrichment of the partially demineralized tissue via formation of redeposited mineral. This re-deposited mineral is normal crystal growth on existing partially demineralized crystals but could also be new crystal formation within the subsurface regions of the enamel or dentin.

In the world of instant access to information via the Internet one can examine the caries process using sites such as the University of Illinois ''Dental Caries (Cariology) Treatment in the New Millennium'' site. This site provides an excellent background on the caries process and what it means for the clinical approach to treatment.

A landmark conference was held by NIH on March 26-28, 2001 entitled, "Diagnosis and Management of Dental Caries Throughout Life." The bibliography for the conference "Diagnosis and Management of Dental Caries" was developed by the National Library of Medicine.

In the consensus conclusions it is stated: "Effective preventive practices, such as the use of fluoride, sugarless products, and dental sealants were reconfirmed, and clinical studies to identify more conservative but more effective non-surgical and surgical approaches are to be applauded. However, it was evident that current diagnostic practices are inadequate to achieve the next level of caries management in which non-cavitated lesions are identified early so that they can be managed by non-surgical methods." The conference also concluded after careful examination of the current evidence at that time that "In spite of optimism about the future, the panel was disappointed in the overall quality of the clinical data set that it reviewed. Far too many studies used weak research designs or were small or poorly described, and consequently, had questionable validity. There was a clear impression that clinical caries research is under-funded, if not undervalued. Moreover, incomplete information on the natural history of dental caries, the inability to accurately identify early lesions and/or lesions that are actively progressing, and the absence of objective diagnostic methods are troubling."

Conclusions drawn from the NIH Caries Management Consensus Conference in 2001 included the following findings regarding effective caries preventive treatments:10

  1. Fluoride. The research data on fluorides in water and dentifrices support their efficacy. The data also support the use of fluoride varnishes. For rinses and gel applications, the evidence is promising but not definitive.
  2. Chlorhexidine. For varnishes and gels, the data are promising. Research data showing effectiveness of chlorhexidine rinses are lacking.
  3. Sealants. The use of pit and fissure sealants is supported by the data.
  4. Combinations. Combinations of chlorhexidine, fluoride, and/or sealants are suggestive of efficacy.
  5. Antimicrobials. Although mutans streptococci is recognized as part of the pathology of caries and, therefore, an antimicrobial approach would seem reasonable, current data are inadequate to support antimicrobial treatments other than chlorhexidine and fluorides, both of which have antibacterial properties.
  6. Salivary Enhancers. Although there are indications that pathologically lower salivary flow, as a consequence of Sjögren's syndrome or as an effect of head/neck radiation treatment or xerostomic medications, is associated with caries, there is no evidence that low normal salivary flow produces a similar outcome.
  7. Behavioral Modification. Most interventions require patient adherence, and current data provide some support for the efficacy of office-based behavioral interventions.

It is important to note that the Consensus Conference also made the following statement, "In the development of caries treatment, dentistry has moved historically from extraction to surgical restoration. Identification of early caries lesions and treatment with non-surgical methods, including remineralization, represent the next era in dental care." We are clearly in an era of dental care in which prevention of demineralization and effective remineralization have taken the spotlight.

Later in that same year, 2001, another consensus report, "Recommendations for Using Fluoride to Prevent and Control Dental Caries in the United States," was published by a working group of the Centers for Disease Control that examined fluoride approaches for caries prevention; and it is evident from this working group’s recommendations that fluoride is an effective approach to caries prevention in a variety of forms.

When used as directed, brushing with fluoride toothpaste is safe and effective, and it represents one of the most cost effective means available for preventing and reversing the caries process. In the US, there are 3 fluoride sources that companies are allowed to use in formulating fluoride toothpaste. These are sodium fluoride (NaF), sodium monofluorophosphate (SMFP) and stannous fluoride (SnF2).

One of the most widely used sources of fluoride in toothpaste is NaF, an ionic form of fluoride that is highly reactive. For this reason, NaF dentifrices need to be formulated with abrasive systems that are fluoride compatible.

SMFP is a covalently bound form of fluoride. In order to release the active fluoride ion from the SMFP, the covalent bond must first be broken by way of either acid or salivary enzyme hydrolysis.   Although it is a bit easier to formulate stable toothpastes using SMFP, delivering the active F ion to the tooth surface requires a two-step process that makes this agent a bit less efficient than its ionic counterparts. 

Probably the most unique of all of the available F sources is SnF2. Like NaF, SnF2 is also an ionic form of fluoride. However, in addition to its fluoridating benefits, SnF2 also provides efficacy against bacterial acids. The first dentifrice accepted by the ADA in 1960 as being clinically effective against caries contained SnF2 (Original Crest® with Fluoristan®).  However, toothpastes at that time were not stable for long periods of time (current products are generally stable for 2-3 years). The most effective of the modern day SnF2 dentifrices are those that have been “stabilized” using proprietary methods. Most importantly, the modern SnF2 dentifrices, in addition to providing anticaries benefits, have also been clinically proven effective against plaque and gingivitis, dental erosion, sensitivity and halitosis.

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Calcium and Phosphate in Remineralization

By definition, calcium is a necessary component of remineralization; whether it is there from previously demineralized mineral, plaque fluid, saliva, or from all sources.There has been a considerable amount of research on the benefits of adding remineralizing agents, such as soluble calcium and phosphate, into oral care products in order to ensure that individuals with low levels of natural salivary minerals are afforded all the benefits of fluoride.11 One of the more interesting approaches is the use of casein phosphopeptide, amorphous calcium phosphate (CPP-ACP) that has shown promise in initial testing as a potential adjunct to fluoride therapy;12 although more work needs to be done to verify the clinical benefits of this type of approach. Fluoride plays a key role in this process by aiding the transformation through the mineral phases, enhancing crystal growth, speeding up the remineralization process, and inhibiting demineralization at the crystal surfaces.

New mineral which is formed will be less soluble than the original mineral and will be either fluorapatite-like or hydroxyapatite-like, with much less carbonate and fewer impurities than the original mineral.

In summary, both calcium and phosphate are needed for remineralization. Importantly, both are generally present in sufficient quantities from endogenous sources, i.e., saliva, for this to occur. If not, supplementation of saliva with additional sources of these key minerals may be helpful; and fluoride enhances the overall process.