Fluoride is integral to the practice of present day dentistry. There is a huge emphasis on prevention currently taught by the dental schools that are training the new cohort of dental professionals. On top of this, the pilots for the new NHS contract suggest that prevention will be the cornerstone of how we treat our patients and how we earn our money.
At the heart of all this is Fluoride, the miracle mineral that helps to “strengthen” and “heal” our teeth. Professionally applied Topical Fluoride bi-annually shows 33% caries reduction in deciduous teeth and 46% reduction in the permanent dentition (Marinho et al 2007), while fluoride of any guise -be it topical, water fluoridation or toothpaste- was shown to reduce coronal caries by 29% and root surface caries by 22% in adults of all ages (Griffin et al 2007).
So we know that it works, but how??
Mechanisms of Fluoride in Caries Prevention.
The structure of enamel is not within the scope of this single article, so a background understanding is required. For now, enamel is made up of packed Calcium Hydroxyapatite(CAP) crystals organised in enamel rods. Fluoride ions are capable of replacing Calcium in these crystals to form a larger, more acid resistant crystal – fluorohydroxyapatite. (FAP)
Systemic Fluoride pre-eruption: The Historical View
Exposure to systemic fluoride whilst teeth are undergoing enamel formation (up to 7 years) incorporates this FAP into the original lattice. Historical thinking suggests that this creates a more “perfect” enamel crystal that would provide life-long caries protection.
In reality, a study comparing erupted teeth in children with and without fluoridated water since birth showed that, although there is greater ppm of FAP in the surface enamel in cases where water fluoridation was available since birth, it was not significant enough to correlate with the decrease in DMFT that was also shown.
If systemic fluoride in the developmental phase is above the optimum level, then there is also a risk of fluorosis, a type of hypomineralisation that presents with a chalky white/mottled appearance.
Topical Application Post-Eruption: The Current Philosophy
- The caries preventing effect of fluoride is currently believed to be it’s role in the active carious process. Where Fluoride is available, it is able to remineralise demineralised tooth tissue and protect the teeth in the following ways:
Typically, CAP crystals are considered small, and contain some impurities. Physically, as FAP is a bigger crystal than CAP, it is able to fill in the gaps between each individual CAP crystal, and thus decreasing the surface area and space available for acid infiltration.
Increased acid resistance
FAP is found to be more acid resistant. Where the critical pH (the point which demineralisation occurs) is 5.5 in CAP, FAP’s critical pH is below 4 (Featherstone et al 1990)
Reservoir of Fluoride
In an environment where there is a high availability of fluoride (topical application, toothpaste, fluoridated water) a Calcium-Fluoride like complex is produced that adheres lightly to the tooth surface. This complex is slightly water soluble, but dissociates at pH 4-5, releasing the fluoride ion. The free fluoride ions are then able to remineralise tooth tissue into FAP.
At high concentrations, fluoride has an effect on the bacterial flora in the mouth. It does this by inhibiting the enzyme enolase. Enolase inhibition affects glycolysis and so acid production of the oral bacterium, namely Streptococcus. Mutans, leading to theoretical anticariogenic properties.
Alteration to Tooth Morphology?
There is a theory that teeth from a fluoridated community tend towards more shallow fissures and rounded cusps, and wider teeth. All physical alterations to the tooth morphology that lends itself to less plaque retention. This, although mentioned in some textbooks, is not a universally accepted benefit of systemic fluoride.
Griffin SO, Regnier E, Griffin PM, Huntley V. 2007. Effectiveness of fluoride in preventing caries in adults (systematic review). J Dent Res, 86:410-415
Marinho VCC, Higgins JPT, Logan S, Sheiham A. 2007. Fluoride varnishes for preventing dental caries in children and adolescents. Cocharane Database of Systematic Reviews Issue 2. Art. No: CD002279, DOI: 10.1002/14651858
Featherstone JD. The science and practice of caries prevention. J Am Dent Assoc 2000;131:887-899.