Fluorides.

Contents
1. Introduction.
2. Background on Formation of Dental Caries.
3. Critical pH and the Stephan Curve.
4. Action of Fluorides in the Control of Dental Caries.
4.1. Decrease Acid Solubility of Enamel
4.2. Enhances Remineralisation.
4.3. Antimicrobial Action.
5. Systemic Vs Topical.
6. Dental Fluorosis.
7. Fluoride Overdose.
8. Skeletal Fluorosis.
9. Methods of Delivery.
9.1. Water Fluoridation.
9.2. Salt Fluoridation
9.3. Milk Fluoridation
9.4. Fluoride Toothpaste.
9.5. Fluoride Tablets.
9.6. Fluoride Mouth Rinses.
9.7. Fluoride Gels & Foams.
9.8. Fluoride Varnish.
9.9. Slow-release devices.
10. References.

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1. INTRODUCTION

Fluoride is the most important and commonly employed tool by many countries around the world, in their fight against caries. Fluorides can be delivered through many vehicles. Some of the most notable forms of fluoride delivery include water, salt and milk. Following the initial success of fluoride in controlling dental caries, fluoride-containing dental products meant to be applied topically were manufactured and sold commercially. Some examples include toothpaste, mouth rinses, professionally applied gels and varnishes. There is an ongoing and active social debate on the delicate balance between the preventive effect of fluoride and the side effects of using fluorides, which will be discussed in greater detail.

2. BACKGROUND ON FORMATION OF DENTAL CARIES

Tooth decay occurs due to the action of specific types of acid-producing bacteria in the presence of fermentable carbohydrates like sucrose, fructose and glucose. Demineralisation occurs when acid-producing bacteria produces lactic acid in the presence of substrates, leading to a loss of tooth structure. On the other hand, remineralisation is also occurring, a process by which minerals are returned to the tooth structure. The tooth constantly undergoes demineralising and remineralising, since the oral cavity is constantly exposed to such carbohydrates and factors which enhance remineralisation. Most of the time, the buffer system in the oral cavity balances out the change in pH, leading to no net loss of tooth structure, but the same cannot be said should the pH drops below 5.5, or otherwise known as the critical pH. When the pH on the tooth surface falls below the critical pH, the demineralisation process would be occurring at a faster pace as compared to the remineralisation process. This would lead to a net loss of tooth structure, which can result in dental caries. Hence, this delicate balance of pH is critical to the preservation of delicate tooth structure.

3. CRITICAL pH AND THE STEPHAN CURVE


As mentioned above, the critical pH value (pH 5.5) is when there will be a net loss of tooth structure and it is important for the oral cavity to be able to respond to sudden pH changes within the oral cavity. The Stephan Curve shows the effects of a sucrose rinse on the pH level in the oral cavity. As seen, the pH level quickly drops below the critical pH minutes after the rinse.

Figure 1.
The Stephan Curve

It is therefore important for the oral cavity to recover itself before the amount of time available for a net loss of tooth structure becomes too long. This is all done by the salivary buffer system in the oral cavity. More information with regard to the Stephan Curve and the buffer systems can be seen at the following website.
Link to: Function of Saliva.


4. ACTION OF FLUORIDES IN THE CONTROL OF DENTAL CARIES

Fluoride combats the formation of dental decay via a few methods as discussed below.
4.1. Decrease acid solubility of enamel

Upon exposure of ionic fluoride, the ionic fluoride may be taken up by enamel to form either fluorhydroxyapatite or calcium fluoride. When the concentration of fluoride is low (i.e. less than around 50ppm) in the oral cavity and with the presence of an acidic environment, the following reaction occurs.

Fluorides. - Cariology

Fluorapatite is more resistant to acid dissolution and dissolves at a lower pH than hydroxyapatite. The important condition in the mouth for the formation of fluorapatite is that the oral fluids, saliva and plaque fluid should be supersaturated with respect to fluorapatite, usually the case when pH is above 4.5. With increasingly low pH, the oral fluids become increasingly unsaturated with fluorapatite and the fluorhydroxyapatite dissolves increasingly with lower pH, usually around pH 4.5.

4.2. Enhances Remineralisation
Link to: Function of Saliva
Link to: Early Caries and Enamel Changes
The Fluorides. - Cariology, or fluorhydroxyapatite, will be formed on the outermost layer of the tooth enamel and forms an integral part of the tissue that will only be degraded if the entire mineral is dissolved. However, under neutral conditions in the oral cavity, the formation of fluorhydroxyapatite is slow (due to the need of H+ ions as shown in the equation above), and is not able to keep pace with the normal wear of the tooth surface. Hence, even with the use of fluoride toothpaste, diligent daily brushing reduces the fluoride content of buccal enamel surfaces as time goes by. On the other hand, in approximal areas, the conditions are more favourable and allows the gradual increase of fluoride content on the enamel surface over a lifetime.

When the concentration of fluoride is high (i.e. above 100ppm), usually when the teeth are given a topical fluoride treatment or exposed to fluoride toothpaste, calcium fluoride is formed via the following reaction.

Fluorides. - Cariology

The higher the fluoride concentration, the more calcium fluoride is formed. Similar to the formation of fluorhydroxyapatite, an acidic environment has a very strong effect on calcium fluoride formation due to the need of H+ ions as shown in the equation above.

Furthermore, at a low pH level, the solubility of enamel increases considerably. The dissolution of enamel provides amounts of calcium for a substantial calcium fluoride formation. It is important to note that calcium fluoride does not only form on sound enamel surfaces. The most important calcium fluoride is that which precipitates in plaque, in enamel porosities and other inaccessible areas. The free ionic calcium concentration in such areas is high and considerable formation of calcium fluoride can be assumed.

However, this calcium fluoride can only act as a temporary storage of fluoride from where the active ion is slowly released. This effect is presumed to last as long as the released fluoride is able to maintain an increased concentration level in the near surroundings. Therefore, calcium fluoride does not thrive well in the oral fluids as the fluids are unsaturated with respect to calcium fluoride. Hence, it disappears within a day on open surfaces but survive for a longer period of time in protected sites or in plaque.

Under normal conditions after topical application, the dissolution of calcium fluoride gives rise to increased fluoride concentrations in saliva but it returns to normal physiological levels after a few days. By then, all the chemical traces of the treatment would have disappeared, only leaving one observation – a reduction in caries incidence in months after the treatment.

4.3. Antimicrobial action

Lastly, fluorides when taken into bacterial cells on the surface of the teeth can interfere with the cellular metabolism of the microbial cells. There are two major pathways by which fluoride affects microbial cells. Firstly, it directly inhibits a variety of enzymes in intact cells, either directly or in the form of metal complexes (Marquis, Clock, & Mota-Meira, 2003). Secondly, it enhances the proton permeability of cell membranes by acting in the form of HF as a transmembrane proton carrier, which discharges changes in pH across the cell membrane. This leads to the inhibition of acid production by intact bacterial cells at low pH values (Marquis, Clock, & Mota-Meira, 2003). In other words, fluoride can attack the oral bacteria that are responsible for producing the acids causing dental caries. Fluoride also binds to the cell walls of bacteria in the oral environment which can serve as a reservoir to continually release fluoride into the fluid bathing the tooth enamel, prolonging the fluoride benefit for enamel remineralisation.

5. SYSTEMIC VS TOPICAL

Previously, it was believed that fluoride needed to be present systemicly to be effective in the control of caries. This is due to the assumption that the method of action of fluorides was due to its incorporation of fluoride into the enamel during enamel formation. It was believed that for fluorapatite to be formed, it was essential that the fluoride ion be present during amelogenesis and hence leading to beliefs that systemic fluoride was essential. However, with more work done using sophisticated enamel biopsy and fluoride analysis techniques, results revealed no relationship between enamel fluoride levels and caries experience. This view was also upheld by further epidemiological proof in that the reduction of caries were found in teeth that have already erupted at the start of fluoridation programmes (Collins & O' Mullane, 1970).

It is now known that early white spot lesion can progress to a cavity, remain static or reverse via remineralisation. The presence of fluoride has been shown to promote the process of remineralisation and the 'healed' lesion has been shown to be more resistant to caries attack (Koulourides, Keller, Manson-Hing, & Lilley, 1980). As mentioned above, fluoride inhibits the process by which cariogenic bacteria metabolise carbohydrates to produce acid (Bowden, 1990), which will lead to decreased incidence of caries.

Therefore, it is now accepted that the method of action of fluorides is mainly topical but fluorides from water supplies also functions systemically when absorbed and incorporated into developing teeth. The systemic action is minor in the prevention of caries compared to the topical action of fluoride.

6. DENTAL FLUOROSIS

Link to: Dental Fluorosis.

Dental fluorosis occurs because of the excessive intake of fluoride. Teeth are generally composed of hydroxyapatite and carbonated hydroxyapatite; when fluoride is present, fluorapatite is created. Excessive fluoride can cause white spots, and in severe cases, brown stains or pitting or mottling of enamel. In its mild form, which is the most common, fluorosis appears as tiny white streaks or specks that are often unnoticeable. In its severe form it is characterized by black and brown stains, as well as cracking and pitting of the teeth. Fluorosis cannot occur once the tooth has erupted into the oral cavity. At this point, fluorapatite is beneficial because it is more resistant to demineralisation. Although it is usually the permanent teeth which are affected, occasionally the primary teeth may be involved. Dental fluorosis occurs between the ages of 3 months to 6-7 years, from the overexposure to fluoride. The earliest manifestation of dental fluorosis is an increase in enamel porosity along the striae of Retzius (Fejerskov, Johnson, & Silverstone, 1974).

7. FLUORIDE OVERDOSE

The lethal dose of sodium fluoride is stated as 5 to 10 grams (
32-64mg/kg body weight for an average adult) and 16mg/kg in children, while the acute toxic dose (single exposure to a toxic substance) is 5 mg fluoride/kg body weight (Whitford, 1996). Acute exposure to fluoride may produce effects of nausea, vomiting, abdominal pain, diaarhoea, fatigue, drowsiness, coma, convulsions, cardiac arrest and death.

Upon ingestion, fluoride initially acts locally on the intestinal mucosa. Hydrofluoric acid can be formed in the stomach which can lead to irritation of the gastro-intestinal system. Following ingestion, the gastro-intestinal system is the earliest and most commonly affected organ system.

Upon absorption of fluoride into the system, fluoride can bind calcium ions and lead to hypocalcemia. Fluoride has direct cytotoxic effects and can interfere several enzyme systems. Fluoride can interrupt oxidative phosphorylation, glycolysis, and neurotransmission. Fluoride also inhibits Na+/K+ - ATPase, which can lead to hyperkalemia due to an extracellular release of potassium. Hypersalivation, vomiting and diarrhoea are some cholinergic signs which can be due to the inhibition of acetylcholinesterase. Seizures may result from both hypomagnesemia and hypocalcemia. Severe fluoride toxicity will result in multisystem organ failure and death will result from respiratory paralysis, dysrhythmia or cardiac failure.

In 1974 a 3-year old child swallowed 45 milliliters of 2% fluoride solution, estimated to be triple the fatal amount, and then died. The fluoride was administered during his first visit to the dentist, and the dental office was later found liable for the death. A typical 3 year old child should weigh approximately 15kg, which means a dose of 75mg of fluoride would be enough to kill the child, a dosage equivalent to consuming 53% of a 130g tube of 1,100ppm fluoride toothpaste.

About 70–90% of ingested fluoride is absorbed into the blood, where it distributes throughout the body. In infants 80–90% of absorbed fluoride is retained, with the rest excreted, mostly via urine; in adults about 60% is retained. About 99% of retained fluoride is stored in bone, teeth, and other calcium-rich areas, where excess quantities can cause fluorosis. Approximately 50% of ingested fluoride is cleared by the kidneys, and hence patients with kidney failure might only be able to excrete 10-20% of ingested fluoride, making them more susceptible to fluoride poisoning.


8. SKELETAL FLUOROSIS

Skeletal fluorosis is a bone disease caused by an excessive consumption of fluoride. It is characterized by hyperostosis, osteopetrosis and osteoporosis (Obel, 1971; Shupe, 1980). The table below shows the different phases of skeletal fluorosis. The preliminary stages can easily be misdiagnosed for rheumatoid arthritis, osteoarthritis or other similar diseases.

Table 1.
Fluorides. - Cariology

According to Kaj Roholm's study of industrial fluorosis (1937), it showed that phases of skeletal fluorosis could occur with a fluoride intake of 0.2 - 0.3mg per kilogram of body weight per day, after 2 years and 5 months for phase I; 4 years and 10 months for phase II and 11 years and 2 months for phase III.


9. METHODS OF DELIVERY

9.1 WATER FLUORIDATION

Fluoride does occur naturally in water supplies at very low concentrations of about 0.1-1.0mgF/l (Smith & Ekstrand, 1996). Schemes were implemented all around the globe following the success of artificially adding fluoride to water supplies in the USA. At the current moment, more than 30 countries and over 250 million people benefit from water fluoridation. Over 100 reports from more than 23 countries are able to support that water fluoridation brings about a reduction in caries in both children and adults. More than 60 studies present data for the deciduous and over 80 for the permanent dentitions (Murray, Rugg-Gunn, & Jenkins, 1991).

All methods of fluoridation, including water fluoridation, create low levels of fluoride ions in saliva and plaque fluid, thus exerting a topical or surface effect. A person living in an area with fluoridated water may experience rises of fluoride concentration in saliva to about 0.04mg/L several times during a day. This intake of fluoride would then aid in the preservation of tooth structure via its method of action as discussed above.

However, there are problems that countries are facing nowadays, from the availability of other fluoride-containing products and the possibility of multiple fluoride exposures resulting in an increased risk of fluorosis, the variability of domestic water consumption within a community, coupled with the knowledge that fluoride need not be ingested to provide substantial benefits, many citizens of the world are questioning the need for water fluoridation. For example, in The Netherlands and Finland, although their pilot schemes of water fluoridation brought about a reduction of caries, their water fluoridation scheme have been suspended for a variety of political and technical issues. Water fluoridation in Singapore achieved good results in the prevention of caries and currently, the water in Singapore is fluoridated with 0.4-0.6mgF/l. In temperate climates teh optimal level of fluoride in drinking water ranges from 0.7 to 1.0 ppm F.


9.2. SALT FLUORIDATION

Salt fluoridation came about in Switzerland after the success of adding iodine to help prevent goiter. More than a 100 million people globally have access to iodized and fluoridated salt. The advantage to this system as compared to water fluoridation is the very fact that consumers have the choice of buying fluoridated or non-fluoridated salt and this leads to relatively little problem with implementation. The fluoride concentration varies from 90-350mgF/kg but the optimal concentration of fluoride in salt to have a preventive effect is 250 ppm F.

9.3. MILK FLUORIDATION

Milk fluoridation was implemented for small groups in some parts of the world, including Eastern Europe, China, the UK and South America (Stephen, Bánóczy, & Pakhomov, 1996). Similar to salt fluoridation, it has a natural advantage over water fluoridation given the fact that it can be targeted directly at segments of a population deemed to be at risk of caries.

Originally, there were concerns over the fact that the fluoride would be inactivated as milk is high in calcium. However, it appears that the majority of fluoride is available in milk up to a concentration of about 5ppm (Edgar, Lennon, & Phillips, 1992). It is also found that fluoride from milk is ultimately absorbed from the alimentary canal, but less quickly than that from water (Whitford, 1996).

The cost of providing fluoridated milk was raised as a problem but quickly put down, as the additional cost of adding fluoride to milk is relatively small, especially when large volumes were involved. Although many studies have been conducted with regard to the effectiveness of milk fluoridation, most of these have been poorly designed with flaws including high attrition rates and interrupted milk supplies. Therefore, there is a need for well-controlled studies on milk fluoridation before this method of fluoridation can be further implemented.

9.4. FLUORIDE TOOTHPASTE
Link to: Oral Hygiene.

Toothpaste today contains 0.1% (1000ppm) fluoride, usually in the form of sodium fluoride or sodium monofluorophosphate. Fluoride toothpaste first became available in 1955 when toothpaste containing 0.4% stannous fluoride or Tin(II) Fluoride was first marketed in the USA. Till date, over 100 clinical trials of toothpastes with different concentrations of fluoride have been conducted (Clarkson, Ellwood, & Chandler, 1993).

Since the introduction of fluoride toothpaste, caries increments in 3-year studies were approximately 20-30% greater in subjects brushing once or less a day compared with those brushing twice or more a day (Chesters, Huntington, Burchell, & Stephen, 1992; O'Mullane, et al., 1997; Ashley, Attrill, Ellwood, Worthington, & Davies, 1999). Similarly, the time of brushing is also subject to some debate. Salivary flow rate and buffering capacity of saliva are reduced during sleep, and hence it is appropriate to relieve the plaque load and boost fluoride levels in the oral cavity before sleep by brushing.

However, one problem about fluoridated toothpaste is the fact that children might tend to swallow toothpaste as some are artificially flavoured and this could cause serious problems as mentioned above with regard to fluorosis of the teeth. Toothpaste may also cause or exacerbate perioral dermatitis most likely caused by sodium lauryl sulfate, an ingredient in toothpaste, as it is suspected that SLS is linked to a number of skin issues such as dermatitis.

9.5. FLUORIDE TABLETS

Tablets containing sodium fluoride were first introduced in the late 1940s to mimic the systemic delivery of fluoride from water fluoridation and hence are used primarily for children in areas without fluoridated drinking water. The dosage should depend on the weight of the subject, but generally for a subject older than 6 years old, 1ppm F/day should be delivered through tablets should fluoridated water not be readily available. The evidence supporting the effectiveness of this treatment for primary teeth is weak. The supplements prevent cavities in permanent teeth. A significant side effect is mild to moderate dental fluorosis. In areas where water fluoridation is provided, it is not advisable for children under the age of 6 years be prescribed fluoride tablets.

9.6. FLUORIDE MOUTH RINSES

The most common fluoride compound used in mouth rinse is sodium fluoride. Current concentrations of mouth rinses is 0.05% sodium fluoride (225 ppm fluoride). The major advantage that fluoride mouth rinses have over water fluoridation is the fact that it ensures maximal topical exposure while minimizing the risk of fluorosis as little fluoride is ingested systemically. It is important to use methods like mouth rinses and toothpaste at different times of the day to maximise the overall efficacy (Blinkhorn, Holloway, & Davies, 1983). However, mouth rinses are not recommended for children under the ages of 6 due to the possibility of dental fluorosis children since young children may not spit properly.

9.7. FLUORIDE GELS & FOAMS

Fluroide gels and foams are available, containing a variety of fluoride levels ranging from 225 ppm F up to 5000 ppm F. It is applied through the use of a mouth tray, and the viscosity of the gels makes it relatively easy to apply and excellent fluoride delivery to the dentition is achieved. Such devices are used for individuals with high caries risk, patients with decreased salivary flow and patients undergoing head and neck radiation. Application is easy and only takes a few minutes.

9.8. FLUORIDE VARNISH

The most widely used varnish is Duraphat varnish, which contains 5% sodium fluoride (22600 ppm F) in suspension in alcohol with a resin system that sets on contact with saliva. It is applied by a trained professional to help prevent decay, remineralise tooth surface and to treat dentine hypersensitivity. There are several advantages with regard to the use of varnishes. Firstly, to appeal to younger patients, fluoride varnishes are available in different flavours and do not have the bitter taste of some fluoride gels. Varnishes are also easily and quickly applied. They dry rapidly and will set even in the presence of saliva. As they do not require the use of fluoride trays, varnishes are suitable for patients with a strong gag reflex. Most importantly, due to the small amounts used and the rapid setting time, there is small or negligible amounts of fluoride ingested, which can ease the worries that children below 6 years old might swallow the varnish and cause fluoride overdose. However, due to the colour and adherence of most varnishes, they may cause a temporary change in the surface colour of the teeth as well as filling materials. This yellowish colour 'stain' fades off with eating and brushing.

9.9. SLOW-RELEASE DEVICES

A good fluoride delivery system would be one that supplies small amounts of fluoride throughout the day. This idea has resulted in devices that slowly release fluoride known as slow-release fluoride devices. These devices can be implanted on the surface of a tooth, typically on the side of a molar where it is not visible and does not interfere with eating.Experiments with such slow-release glass materials retained on the buccal surface of molar teeth have shown some promise (Toumba & Curzon, 1993). The two main types are copolymer membrane and glass bead. These devices are effective in raising fluoride concentrations and in preventing cavities, but they have problems with retention rates, that is, the devices fall off too often.

Figure 2.
http://dubois.vital.co.uk/dynamic/filelibrary/dentistry/TOUMBAPIC.jpgFluorides. - CariologyFluorides. - CariologyFluorides. - CariologyFluorides. - Cariology
(Pessan, Al-Ibrahim, Buzalaf, & Toumba, 2008)


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10. RESOURCES

1. Ashley, P. F., Attrill, D. C., Ellwood, R. P., Worthington, H. V., & Davies, R. M. (1999). Toothbrushing habits and caries experience. Caries Res, 33(5), 401-402.

2. Blinkhorn, A. S., Holloway, P. J., & Davies, T. G. (1983). Combined effects of a fluoride dentifrice and mouthrinse on the incidence of dental caries. Community Dent Oral Epidemiol, 11(1), 7-11.

3. Bowden, G. H. (1990). Effects of fluoride on the microbial ecology of dental plaque. J Dent Res, 69 Spec No, 653-659; discussion 682-653.

4. Chesters, R. K., Huntington, E., Burchell, C. K., & Stephen, K. W. (1992). Effect of oral care habits on caries in adolescents. Caries Res, 26(4), 299-304.

5. Clarkson, J. E., Ellwood, R. P., & Chandler, R. E. (1993). A comprehensive summary of fluoride dentrifice caries clinical trials. Am J Dent, 6(Special Issue), S59-S106.

6. Collins, C. K., & O'Mullane, D. M. (1970). Dental caries experience in Cork City schoolchildren aged 4-11 years after 4 1/2 years of fluoridation. J Ir Dent Assoc, 16(5), 130-134.

7. Edgar, W. M., Lennon, M. A., & Phillips, P. C. (1992). Stability of fuoride in milk during storage in glass bottles. J Dent Res, 71(Special Issue), 703.

8. EPA (2009). Skeletal fluorosis is a crippling bone disease caused by fluoride. Retrieved 19 October 2009, from http://www.fluoridation.com/skeletal.htm

9. Fejerskov, O., Johnson, N. W., & Silverstone, L. M. (1974). The ultrastructure of fluorosed human dental enamel. Scand J Dent Res, 82(5), 357-372.

10. Fejerskov, O., & Kidd, E. (2008). Dental Caries: The Disease and its Clinical Management. Oxford: Blackwell Munksgaard.

11. Fejerskov, O., Nyvad, B., & Larsen, M. J. (1994). Human experimental caries models: intra-oral environmental variability. Adv Dent Res, 8(2), 134-143.

12. Hellwig, E., & Lennon, A. M. (2004). Systemic versus topical fluoride. Caries Res, 38(3), 258-262.

13. Koo, H. (2008). Strategies to enhance the biological effects of fluoride on dental biofilms. Adv Dent Res, 20(1), 17-21.

14. Koulourides, T., Keller, S. E., Manson-Hing, L., & Lilley, V. (1980). Enhancement of fluoride effectiveness by experimental cariogenic priming of human enamel. Caries Res, 14(1), 32-39.

15. Marquis, R. E., Clock, S. A., & Mota-Meira, M. (2003). Fluoride and organic weak acids as modulators of microbial physiology. FEMS Microbiol Rev, 26(5), 493-510.

16. Murray, J. J., Rugg-Gunn, A. J., & Jenkins, G. N. (1991). Fluorides in caries prevention (3rd ed.). Oxford, Boston: Wright.

17. NCL (2009). Stephan Curve: the basics. Retrieved 19 October 2009, from http://www.ncl.ac.uk/dental/oralbiol/oralenv/tutorials/stephancurves1.htm

18. O'Mullane, D. M., Kavanagh, D., Ellwood, R. P., Chesters, R. K., Schafer, F., Huntington, E., et al. (1997). A three-year clinical trial of a combination of trimetaphosphate and sodium fluoride in silica toothpastes. J Dent Res, 76(11), 1776-1781.

19. Obel, A. L. (1971). A literary review on bovine fluorosis. Acta Vet Scand, 12(2), 151-163.

20. Pessan, J. P., Al-Ibrahim, N. S., Buzalaf, M. A., & Toumba, K. J. (2008). Slow-release fluoride devices: a literature review. J Appl Oral Sci, 16(4), 238-246.

21. Robinson, C., Connell, S., Kirkham, J., Brookes, S. J., Shore, R. C., & Smith, A. M. (2004). The effect of fluoride on the developing tooth. Caries Res, 38(3), 268-276.

22. Roholm, K. (1937). Fluorine Intoxication: A Clinical-Hygienic Study. Copenhagen.

23. Shupe, J. L. (1980). Clinicopathologic features of fluoride toxicosis in cattle. J Anim Sci, 51(3), 746-758.

24. Smith, F. A., & Ekstrand, J. (1996). The occurrence and the chemistry of fluoride. In O. Fejerskov, J. Ekstrand & B. A. Burt (Eds.), Fluoride in dentistry (pp. 20-21). Copenhagen, Munksgaard.

25. Stephen, K. W., Bánóczy, J., & Pakhomov, G. N. (1996). Milk fluoridation for the prevention of dental caries. Geneva: World Health Organization.

26. Ten Cate, J. M. (2004). Fluorides in caries prevention and control: empiricism or science. Caries Res, 38(3), 254-257.

27. Toumba, K. J., & Curzon, M. E. (1993). Slow-release fluoride. Caries Res, 27 Suppl 1, 43-46.

28. Whitford, G. M. (1996). Soft tissue distribution of fluoride. In H. M. Myers (Ed.), The Metabolism and Toxicity of Fluoride (2nd ed., Vol. 16, pp. 30-45). Basel, Switzerland: Karger.

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