Introduction: Like other aspects of health care, dental products containing fluoride have been around for so long that they are seldom questioned by health professionals. Over 35,000 articles have been published on various modalities and effects of fluoride, making fluoride well studied but often boring. It works; let’s move on is how dentistry and medicine largely view it. But, when the popular media, internet or patients raise questions, it might be good to have a short reference manual at hand. Several good resources are available and listed in the appendix, but here is a quick and dirty summary of fluoride when you need a quick fact. Just make sure you combine it with another controversial substance – Caffeine.
History: Fluoride has a colorful history that started in Colorado Springs in 1901. A young dentist, Dr. McKay, moved to town and found two things that alarmed him. One, everyone in Colorado Springs had brown mottled teeth. Second, almost no dental decay existed. At a time when dental decay was rampant back east, this indeed was a remarkable finding. He was able to attract the attention of a prominent dental researcher who then found other areas in the country where the condition was repeated. Soon the correlation between brown stains and lack of caries was established.
Later the cause of the staining was found to be high levels of fluoride in various water sources. The director of NIH at the time, Dr. Trendley Dean, then made the intuitive leap that fluoride at low enough levels may reduce decay rates but not produce unsightly brown staining. In a feat of epidemiological research, he found the junction that maximized caries prevention while minimizing fluorosis. Dean’s fluoride level of 1ppm was later tested in Grand Rapids Michigan in 1944. Eleven years later caries rates where shown to be reduced 60% with no significant side effects. The age of fluoridation was born. In 1964, Stanly Kubrick’s Dr. Stangelove made water fluoridation a communist plot. In 1967 Crest introduced fluoride toothpaste. The rest is history…
Mechanism of Action: Fluoride has two modes of action. When ingested, approximately 50% of fluoride is deposited into the teeth and bones, 50% is excreted. In bones, fluoride moves in and out as bones remodel. In teeth, fluoride incorporates into enamel and does not remodel once it is formed. As the tooth if formed and calcifies, fluoride is incorporated into the crystal structure of the enamel along the protein scaffolding laid down by specialized cells called ameleoblasts. At 1 ppm, fluoride does not interrupt the crystal formation of the tooth, but at levels above that some disruptions in crystal formation begin to occur. At 2ppm, visible disruptions in tooth structure can occur, and at 4ppm and above, ameleoblast cell alteration causes significant amounts of disrupted protein matrix.
The enamel structure warps, loses its translucence and may chip and picks up food stain. This condition, coined Fluorosis runs a spectrum of severity, but does not make the tooth more susceptible to decay. Fluoride only works systemically while the teeth are forming. Fluoride does not cross the placental barrier and is a trace element in breast milk, so rarely are primary teeth effected, Permanent teeth begin calcifying at birth for first molars, about 2 years for central incisors and about 4-6 years for second premolars. Introduction of systemic fluoride at various ages and at various amounts then accounts for the level of caries resistance and fluorosis seen in the adult dentition. Often ingestion of fluoride toothpaste during the toddler years affects the front permanent teeth esthetics. From the early studies out of Grand Rapids, it is obvious systemic fluoride plays a role in caries resistance. But in this day and age when water fluoridation is available to about 60% of the US and fluoride toothpaste is ubiquitous, considerable debate exists about how much systemic fluoride plays in modern day caries resistance.
The second mode of action is Topical. Topical fluoride describes the fluoride ion being present in the mouth and available to tooth surfaces. On the tooth surface a constant demineralization is occurring due to plaque acid and to a lesser extent acidic foods (which are usually well buffered by the saliva). In these areas, the negatively charged fluoride ion is attracted and serves as a catalyst to begin a remineralization of tooth structure with salivary calcium and phosphates. Enamel is formed of hydroxyapatite crystals, but in the presence of fluoride ion, the hydroxy ion is substituted and a fluoroapatite crystal is formed on the outer enamel. The fluoroapatite crystal is less soluble and more acid resistant, thereby becoming more caries resistant. The more often fluoride is present in the saliva, the more caries resistant teeth become. This is especially true of newly erupted teeth that have not yet reached full mineral content. These teeth are especially prone to decay, but benefit the most from topical fluoride.
Topical fluoride comes in many forms. Water fluoridated at 1ppm will increase salivary fluoride 100 to 1000 times and remain high for 1 -2 hours. The more water washing over the teeth the longer the balance is tipped to remineralization. The same action to an even greater salivary concentration occurs with regular use of toothpastes, mouthwashes or professionally applied fluorides. While higher dosed fluoride modalities will target and help to reverse hypomineralized tooth structure or “white spots,” frequency of exposure to fluoride will lessen caries rate most dramatically.
Fluoride varnish works on a microscopic level to form Calcium fluoride microcrystals visible as small globules on the surfaces of fluoride treated teeth. “This calcium fluoride may serve as a source of fluoride for the formation of fluoroapatite, and the latter phase is formed when pH drops in plaque, not during topical application.” (Rolla et al Adv Dent Res 1994) Since the amounts and size of calcium fluoride globules relates to pH, an acidulated fluoride gel applied for a short period of time likely produces the same reservoir of calcium fluoride as a varnish that sticks to teeth longer. Studies comparing the caries reduction of the two modalities show no clear superiority. Fluoride varnish that is induvudualy packaged, dispensed in metered dosages and applied on a three to six month basis may have advantages over a weekly fluoride rinse or professionally monitored, semi annual gel trays. The issue on modality delivery is nore likely a choice of convenience for the provider and compliance for the patient.
Lastly, Fluoride comes in different salt solutions so it can remain stable until solublized into the active fluoride ion in water or saliva. Neutral Sodium Fluoride is most common in toothpastes because it has long shelf life and can be easily flavored. Stannous Fluoride is less stable and more disagreeable in taste, but the Stannous or Tin ion has shown some trends towards reducing gum inflammation. Other compounds exist, but the active ingredient remains the fluoride ion.
Dosages: Like any nutrient or drug, fluoride has levels of effectiveness and toxicity. The National Academy of Sciences has set the adequate intake or AI of fluoride to be 0.05mg/kg/day. Based on 1ppm fluoridated water and minimal intake of fluoride from dental products, most children will approximate this AI based on caloric needs and fluid intake based on weight.
The Food and Nutrition Board of the Institute of Medicine has set an upper limit or UL of dietary fluoride for children. This UL is set to prevent moderate (cosmetically objectionable) fluorosis. The UL is essentially double the normal fluid intake for children. In extremes of diet where most of the caloric intake is coming from beverage manufactured with fluoridated water, fluorosis may occur.
Fluoride toxicity may occur at approximately 5mg/kg in a single dose for children. This intake, usually from eating toothpaste, results in stomach ache, vomiting, sweating and hypersalivation. Although the evidence is lacking, a lethal dose is around 32mg/ kg. At this dosage death would result by cardiac arrest in 2-4 hours. First aid would be to induce vomiting and give a binding agent for fluoride, milk in the home or activated carbon lavage in the hospital. A typical 4.3 oz child’s toothpaste contains about 132 mg of fluoride. If a year old child weighing 10kg ate the entire tube at a sitting (without somehow vomiting her guts out at half a tube), this would be less than half the potential fatal dose. The American Dental Association recommends no product that is bulk packaged (toothpaste, mouthwash) contain more than a total of 264mg of fluoride.
For Fluoride varnish, a single dose application contains 0.5mg. About half of that, 0.25mg, is recommended for children less than three years old.Fluoride Controversies: As long as fluoride has been used to prevent dental decay, groups have been protesting its use and assigning negative side effects. Many, but of course not all possible side effects have been studied. Below is a short synopsis of those findings.
Cancer- A well known antifluoride person, Yiamouyiannis, gained popularity claiming fluoride caused cancer in the 1970’s. Studies by Yiamouyiannis claimed fluoridated cities had higher cancer rates, and these widely advertised studies gained the ear of congress. Multiple more rigorous studies found severe flaws in the original study and no correlation of fluoride to cancer deaths. But, in 1990, four male rats died of bone cancer in an NTP study after ingesting 79ppm F water. The study authors concluded “equivocal evidence” linking high fluoride to bone cancer. This ignited a fire storm until the study was repeated with no recurrence of that cancer rates. A special committee appointed by the US Public Health Service later reached agreement that all available human and rat studies showed no detectable cancer risk from fluoride intake.
Bone Fracture or Osteoporosis – Widely circulated images of Indian adults with crippling skeletal fluorosis led to the much publicized view that dental fluorosis was an early indicator for skeletal fluorosis causing bone fractures and osteoporosis. In some parts of India and other areas, unregulated water containing fluoride in excess of 25 ppm does cause bone fragility, this condition is unknown in the US. A town in Texas reported a water fluoride content of 10 ppm water fluoride, but bone scans of lifelong residents showed only the barest increase in bone density. In fact, elderly people where prescribed 30-60mg of fluoride per day to increase bone density in the 1970’s. Fluoride did increase bone density but did not prevent bone fractures. Extensive reviews of the literature show in fact no clear evidence to say fluoride exposure through water and other sources either causes or prevents hip fractures. If there is an effect one way or the other it is too small to be detected in the 14 epidemiologic studies to date.
Thyroid Function, Kidney Function, Autism, Down Syndrome, child development – At various times in our history, claims have been made to suggest exposure to fluoride caused various maladies. No review of the literature suggests any such cause and several studies specifically looking for links have found no evidence to indicate fluoride as a toxin related to any disease. Many books or non peer reviewed articles in circulation dating back the 1950’s claim other wise. Such references may contain evidence from anti fluoride proponents including: The Natick Study 1997, Thyroid Power harperesource 2001, Drs. Richard and Karilee Shames, Dr. R.D. Masters and M. Coplan, Dr. Hardy Limeback.
Professional Topical Fluoride Applications – Clinical Efficacy and Mechanism of Action. B Ogaard, L Seppa, G Rolla. Advances in Dental Research 8(2): 190-201. July 1994.
Dentistry,Dental Practice and the Community 6th Ed. Brain Burt and Stephan Eklund. Elsevier Sanders. St. Louis. 2005
Fluoride Technical Study Group report 1/10/2003. Fort Collins. Archived with the City of Fort Collins Utilities.