October/November 2015 Teacher's Guide for Tooth Decay: a delicate Balance Table of Contents

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Erosion caused by chronic vomiting in bulimia.
Image source: Copyright © 2003 Lippincott Williams & Wilkins

Acid reflux and GERD (Gastroesophageal reflux disease) can cause stomach acid (HCl) to be regurgitated up the esophagus back into the mouth. This exposes the enamel of teeth to the highly corrosive mix, which increases the severity of enamel erosion.

Another area of concern related to tooth erosion is eating disorders like anorexia and bulimia. Repeated vomiting by sufferers of these disorders exposes tooth enamel to the highly acidic environment of the hydrochloric acid in stomach acid. If this were to occur on a regular basis, it can result in severe damage to tooth enamel—long-lasting damage that can’t be undone, even long after the disorder has been effectively treated.
Note in the photo above the almost complete lack of enamel on parts of the teeth. The yellow color is the dentin showing through what’s left of the enamel. Also note that the primary area of erosion is on the posterior surfaces, where the regurgitated stomach content is most likely to come in contact with the teeth.
Friction and erosion
And acids aren’t the only problems our teeth face in their quest to maintain the hydroxyapatite equilibrium and avoid cavities. We also create some of our own problems that erode our teeth. Normal (and abnormal) chewing can wear down teeth, as can bruxism—grinding your teeth, especially at night. Here are some other ways we erode our teeth.

  • Attrition. This is natural tooth-to-tooth friction that happens when you clench or grind your teeth such as with bruxism, which often occurs involuntary during sleep.

  • Abrasion. This is physical wear and tear of the tooth surface that happens with brushing teeth too hard, improper flossing, biting on hard objects (such as fingernails, bottle caps, or pens), or chewing tobacco.

  • Abfraction. This occurs from stress fractures in the tooth such as cracks from flexing or bending of the tooth.

  • Corrosion. This occurs chemically when acidic content hits the tooth surface such as with certain medications like aspirin or vitamin C tablets, highly acidic foods, GERD, and frequent vomiting from bulimia or alcoholism.

While these causes of tooth erosion result in the wearing down of tooth surfaces, they don’t usually result in dental caries, as they generally only erode already exposed areas of enamel, like the bite surfaces of teeth; they don’t usually penetrate the enamel down to the dentin, where cavities can really go wild. (In extreme cases, these processes can result in opening areas to bacterial infection, particularly along the gum line.)
Tooth decay
OK, so if you stay away from acidic drinks, and don’t grind your teeth, you can avoid tooth decay, eh? Not so fast. While all of that may minimize tooth erosion, we haven’t even begun to talk about what actually makes teeth decay—Streptococcus mutans. S. mutans is a bacteria that lives in your mouth and absolutely loves sugars, especially sucrose. This bacteria attaches as individual cells, produces a slime-like material to help it adhere to the tooth, and then reproduces on the enamel surface to form a biofilm consisting of hundreds of cells.
The biofilm is extremely resistant to being removed. The S. mutans bacteria within this biofilm are able to cleave sucrose, a disaccharide, (from food we eat) into the monosaccharides glucose and fructose. S. mutans ferments the fructose for use as an energy source for its own growth. The glucose is polymerized into the slime- or glue-like material that attaches to teeth and becomes the biofilm base for dental plaque—the growth of colonies of bacteria.
Subsequent depolymerization of the dextran polymer within the biofilm by the bacteria can result in fermentation of the glucose monosaccharide to produce lactic acid. Some of this acid is trapped within the plaque matrix, confined close to the tooth enamel, where saliva can’t reach to wash the acid away, exposing the enamel to relentless erosion. And note that the Ka for lactic acid is 1.4 x 10–4, more acidic than either citric or carbonic acids. (Bad news for tooth enamel) (http://microbewiki.kenyon.edu/index.php/Streptococcus_mutans)
And then, just when you thought there was only one species responsible for tooth decay, here comes another one (or, actually, more).
While streptococci family bacteria (e.g. Streptococcus mutans) are the main cause of tooth decay, other varieties of microbes can cause dental caries, but to a lesser extent. For example, although considered beneficial, some Lactobacillus species have been associated with dental caries. The Lactobacillus count in saliva has been used as a "caries test" for many years. This is one of the arguments used in support of the use of fluoride in toothpaste. Lactobacilli characteristically cause existing carious lesions to progress, especially those in coronal caries. The issue is, however, complex as recent studies show probiotics can allow beneficial lactobacilli to populate sites on teeth, preventing streptococci pathogens from taking hold and inducing dental decay.
Lactobacilli, like S. mutans, produce lactic acid in their fermentation of simple sugars, adding that acid to the stores of acid produced by other bacteria and tucked away in dental plaque, waiting there to cause tooth decay.
Left unchecked (or unbrushed), erosion of tooth enamel will eventually lead to an opening (cavity or carie) in the enamel which will continue to grow until it reaches the dentin. At this time, one may feel twinges of pain when hot or cold drinks hit the cavity. Also at this point, the rate of decay progresses rapidly, as dentin is softer and more susceptible to the effects of acid (and bacteria). By this time, one will probably begin to feel more prolonged (but possibly still mild) pain. Ultimately, the decay will reach the pulp of the tooth, at which point the pain may be unbearable. An abscess may form within the pulp and dentin, making the whole area of the mouth near the tooth painful. To avoid this scenario, one must practice good oral hygiene.
More on tooth decay & hydroxyapatite equilibrium
It is interesting to note that tooth formation and decay is almost identical in animals to that in humans. The functions of all the parts of the tooth are identical in both, with slight variations in the enamel. Dogs typically suffer from tooth decay much less frequently than humans because saliva in dogs has a much higher pH than in humans. The less acidic environment in dogs’ mouths results in less demineralization of the enamel.
Teeth are in a constant state of demineralization and remineralization. Acidic conditions increase the rate of demineralization, leading to cavities or dental caries. At a pH of 5.5 or lower, demineralization occurs at a more rapid rate than remineralization. Many foods are in this range of pH, so without remineralization, eating these foods would automatically result in tooth decay.
The constant battle between demineralization and remineralization can be considered chemically to be an equilibrium system which is under constant stress.
The enamel of teeth is made of hydroxyapatite (also called hydroxylapatite), empirical formula Ca5(PO4)3OH. The formula is usually written as a dimer, Ca10(PO4)6(OH)2, to denote that the unit cell contains two empirical formula units. Hydroxyapatite forms a 3-dimensional crystal structure which is very hard and durable.
Demineralization of hydroxyapatite occurs in acidic conditions; e.g., when bacteria produce acids from their metabolism of ingested sugars. The primary acid produced is lactic acid, along with smaller amounts of formic, acetic and succinic acids, all of which act to dissolve the enamel of teeth.
Ca10(PO4)6(OH)2(s) + 8 H+(aq) → 10 Ca2+(aq) + 6 HPO42–(aq) + 2 H2O(l)
In a less acidic (more basic) environment, remineralization occurs:
10 Ca2+(aq) + 6 HPO42–(aq) + 8 OH(aq) → Ca10(PO4)6(OH)2(s) + 6 H2O(l)
Demineralization and remineralization occur at different rates throughout our lives. In children, remineralization occurs more rapidly than demineralization. In adults, the two reactions occur at roughly equal rates (equilibrium), while in older adults, demineralization can occur faster than remineralization, resulting in the slow loss of tooth enamel and the subsequent possible loss of the tooth. Of course, at any point in our lives when we have significant plaque build-up, we may suffer increased rate of demineralization.
As shown above, lower pH (higher acidity) enhances demineralization. When plaque builds up, the bacteria in the plaque supply H+ ions in close proximity to the enamel. The H+ ions react with the OH- ions from the hydroxyapatite, resulting in destruction of the crystal structure, weakening the tooth enamel. And as the OH- ions are consumed, they reduce the rate of the remineralization reaction (Le Châtelier’s principle), furthering the effect.
Normal pH in the mouth is about 6.8. Demineralization becomes the dominant process when the pH drops below 5.5. This can occur within minutes of drinking a sugar (or high fructose corn syrup) based soft drink and can last for about 10 minutes. Saliva will gradually wash away the acidic material and return the mouth environment back to normal within about an hour. Of course, that means that teeth are exposed to an acidic environment for most of that time, promoting demineralization. Brushing teeth right after eating can remove the acid and return the mouth to its normal pH immediately.
(ChemMatters Teacher’s Guide. October 2011, pp 59–60)
More on effects of brushing and flossing
Brushing teeth (if done right) effectively removes the sugars, acids, plaque and bacterial build-up that would otherwise ensure dental erosion and caries formation. Thus it is an effective weapon against cavity formation. But, as noted above, timing of brushing is important. Acidic drinks can leave the enamel softened for some time after drinking them. So, it’s important to wait, perhaps ½ to 1 hour after drinking, before brushing.
It’s also important not to brush too hard, or particles of the enamel might be ground away by the bristles of the brush. Likewise, it’s better to use a soft bristle toothbrush, for the same reason.
And you need to be sure to use a toothpaste that is not too abrasive, so that it does not remove enamel when you brush. Another ingredient to be concerned about is the sweetener used to make the tooth paste palatable (but not too palatable, or it might be swallowed).

“Toothpaste ingredients do typically include a sweetener. However, because of the process described above, sucrose is not a reasonable choice. Toothpastes commonly use two of the artificial sweeteners … saccharin and aspartame. Some products specifically advertise that they do not use artificial sweeteners. One substitute is the use of essential oils, such as spearmint.”

(ChemMatters Teacher’s Guide. October 2011, p 73)
One much overlooked—and underrated—procedure that can contribute significantly to decay prevention is flossing. Dentists almost invariably ask patients whether they floss regularly, because they know the importance of this tool in the oral health arsenal. Flossing essentially picks up where tooth brushing leaves off.
Brushing cleans plaque off the anterior and posterior surfaces, as well as the “nooks and crannies” available to its bristles; however, the brush can’t clean surfaces between the teeth, or the tooth surfaces down at the gum line. So brushing only cleans about 60–65% of enamel surfaces in your mouth. The other 35–40% of the enamel, where the tooth brush can’t reach, is susceptible to plaque build-up, which continues to erode the enamel (remember, bacteria in the plaque produce acid), and that plaque eventually becomes calcareous tartar. This hard material can only be removed by the dentist scraping your teeth, a process called scaling. So, flossing can prevent acid erosion between your teeth and along the gum line, and it can prevent your needing this somewhat painful professional scaling procedure.
As you know, plaque on teeth results in the tooth enamel being held in constant contact with acid produced by bacteria in the plaque, hastening enamel erosion. Along the gum line, those same bacteria can also cause infection resulting in gingivitis, an inflammation of the gums and, eventually, if not treated, to periodontal disease which could lead to teeth loss.
Is gum disease really worrisome? Studies have shown a link between gum disease and serious medical conditions, such as heart disease and stroke, and low birth-weight in babies. So, anything we can do to prevent gum disease seems to be worthwhile, and flossing is high on that list.
According to Rockside Family Dental Care, there are 10 reasons one should floss daily.
In less than one minute per day you can accomplish the following health benefits:

10 Reasons to Floss!!

  1. Prevent Decay

  2. Prevent gum disease

  3. Fresher breath

  4. Whiter smile (less stain)

  5. Younger smile (less gum recession)

  6. Less dental expense

  7. Less dental pain

  8. Less time away from work or family life

  9. Healthier Heart (bacteria from gum disease has been linked to certain types of heart disease).

  10. Maintains health/condition of dental restorations

OK, so you’ve seen the light; flossing is important and you will do it from now on. What’s the right way, floss before or after brushing? It really doesn’t matter, but dentists point out that if you floss first and then use a fluoride toothpaste to brush, the fluoride has a better chance of finding its way to the enamel between your teeth and at the gum line to better protect your teeth from decay.
And let’s not forget about mouthwashes. These decay-preventers help by killing the germs that cause plaque and acid erosion. Most mouthwashes also contain fluoride, which helps to remineralize tooth enamel and make it stronger (fluorapatite is more stable than hydroxyapatite). So mouthwashes pack a “double whammy” for tooth decay prevention.
More on fluoride treatment of teeth
This excerpt from an earlier (1986) ChemMatters article discusses the use and benefits of using fluoride mouthwash, including the hydroxyapatite equilibrium and fluoroapatite addition to tooth enamel.
One of the best ways you can … strengthen your teeth—is by using mouthwash, which kills the bacteria in your mouth. One key ingredient in many mouthwashes is fluoride, which is known to strengthen tooth enamel (Fig. 6). Fluoride (F) is the ionic form of fluorine. It forms when a fluorine atom gains an electron. Fluoride does not exist by itself, but it can be found in compounds, such as sodium fluoride (NaF), which is present in many toothpastes and mouthwashes. When this compound is dissolved in water, the fluoride ions are free to move.
Fluoride ions prevent tooth decay by strengthening the enamel. The primary compound found in tooth enamel is a strong, insoluble mineral called hydroxyapatite [Ca5(PO4)3(OH)]. Hydroxyapatite contains positive ions (Ca2+) and negative ions (PO43– and OH), which are attracted to each other to form the crystalline structure of hydroxyapatite.
The bacteria present on our teeth produce acids that cause hydroxyapatite to break apart—a process called demineralization:
Ca5(PO4)3(OH)  5 Ca2+ + 3 PO43– + OH
A certain amount of demineralization is normal. But it is also normal for the reverse process, remineralization, to occur:
5 Ca2+ + 3 PO43– + OH Ca5(PO4)3(OH)
If too much bacterial acid is produced, demineralization can outstrip mineralization, leading to a cavity. How does this happen? When acids are present in a solution, they dissolve to produce hydrogen ions (H+). In the mouth, as bacteria produce acids, the amount of hydrogen ions builds up. These ions combine with the hydroxide ions produced during demineralization to form water:
H+ + OH H2O
But hydroxide ions are essential to remineralization, so their neutralization by hydrogen ions causes remineralization to slow down. The hydroxyapatite on the surface of the teeth keeps dissolving, ultimately leading to tooth decay. Fluoride ions present in mouthwashes help the enamel to remineralize. They accumulate on the surface of the enamel, thus creating a barrier that prevents bacterial acids from reaching the enamel. Also, the fluoride ions attract calcium ions, ultimately changing hydroxyapatite into fluoroapatite [Ca5(PO4)3F], which is stronger than the original hydroxyapatite.
(Rohrig, B. Demystifying Gross Stuff. ChemMatters, 2011, 29 (3), pp 13–14)
“The fluoride ion takes the place of the OH during the remineralization process…

The modified enamel, called fluorohydroxyapatite, is more resistant to acid. The F does not substitute for all of the OH; even a small uptake of fluoride makes the enamel less susceptible to decay.”

(Yohe, B. Tooth Paste. ChemMatters, 1986, 4 (1), pp 12–13)
Fluoride, present either through fluoridated water or through fluoride-enhanced toothpaste or mouthwash, becomes important in the demineralization/remineralization equilibrium because when fluoride ions enter the equilibrium, they produce fluorohydroxyapatite (fluoroapatite), which is harder, more stable and more resistant to acid decay than naturally-occurring hydroxyapatite.
10 Ca2+(aq) + 6 HPO42(aq) + 6 OH(aq) + 2 F(aq) → Ca10(PO4)6F2(s) + 6 H2O(l)

  1. Fluoride ions (F–) replace hydroxyl groups (OH–) in hydroxyapatite to form fluorapatite in the tooth enamel.

  2. A portion of the apatite crystal lattice is depicted showing the replacement of hydroxide for fluoride.

Adapted from: Posner, 198520

Here’s why fluoride ions are so successful at replacing hydroxide ions in hydroxyapatite.

Fluoride ions are very similar chemically to the hydroxide ion. Their sizes are similar, as are their chemical reactivities. (Recall oxygen and fluorine positions on the periodic table, and their atomic structures.) This makes it easy for the fluoride ion to replace the hydroxide ion in hydroxyapatite [see diagram at right]. And the fluoroapatite produced is actually more stable than the original hydroxyapatite.
In addition to its role in the remineralization process, fluoride also reduces/ prevents cavities by targeting the metabolic processes of bacteria to actually reduce the amount of acid secretions by bacteria in the mouth. This has the effect of reducing the amount of food consumed and thus the amount of acid produced by the bacteria. With a less acidic environment, there is less demineralization. This process seems to be secondary to fluoride’s role in the remineralization process and the formation of fluoroapatite in tooth enamel, however.
Research has shown that treatment of tooth enamel, bone and calcium phosphate with fluoride all result in lower solubility than just the associated calcium compounds. It is believed that this lower solubility in fluoroapatite is the main (and possibly, the only) factor in the slower rate of demineralization of fluoride-treated enamel. Studies with people exposed to fluoridated water supplies have shown reduced incidence of cavities.
Studies have also been done that also show that fluoride has a greater effect on areas of the tooth enamel already subject to cavity formation—areas of teeth where demineralization has already begun—than in areas where the surface remains intact. This makes sense, since areas of demineralization are areas where the crystal structure has already been compromised and therefore is more prone to rebuilding as fluoride ions come in contact with the greater surface area of “jagged” edges of tooth enamel. These areas of greater fluoride uptake (measured at twice the fluoride concentration of intact enamel) were tested and shown to be much less soluble than intact enamel in the same teeth.


When enamel is exposed to fluoride, it is also possible to form calcium fluoride, CaF2. Calcium fluoride is less soluble than sodium or stannic fluorides, so it precipitates on the enamel. This can act as a source of fluoride ions, especially in acidic conditions, when demineralization would normally occur. Then the fluoride ion would be right there to join with the hydroxyapatite structure to form fluoroapatite. It can also increase the concentration of fluoride in the saliva, thereby reducing the bacterial metabolism of sugars, the acidity of the environment and hence, demineralization. (http://www.intelligentdental.com/2011/08/28/fluoride-toothpaste/)
(The above quote, pp 16–18, was reproduced from ChemMatters Teacher’s Guide, October 2011, p 60)
A common means of adding fluoride to our “diet” in the U.S. is by the fluoridation of municipal water supplies. But not everyone believes that water fluoridation is a good idea, including many European countries. The Fluoride Action Network provides some compelling data (in this graph) that they claim refutes the idea that water fluoridation helps fight tooth decay. The data was gathered from the World Health Organization (WHO). (http://fluoridealert.org) (http://fluoridealert.org/studies/caries01/)http://www.fluoridealert.org/uploads/who_data01.jpg
[Note: DMFT means decayed, missing or filled teeth.]
The graph shows that the general trend across all countries is that the number of DMFTs is decreasing, even in those countries that do NOT fluoridate their water. The Fluoride Action Network claims that this substantial decrease is due to fluoridation of toothpastes and mouthwashes, and they argue therefore that fluoridation of water systems is not warranted.
The http://fluoridealert.org Web site provides more data on specific countries, and actively advocates for the removal of fluoridation from our water supplies.
More on the effect of too much fluoride
OK, fluoride helps prevent tooth decay, but is fluoride safe?
The oral LD50 value of sodium fluoride, the active ingredient in toothpaste, is 31 mg/kg. This substance is considered highly toxic and is classified as a poison, yet we put it into our mouths every day. But we swallow such small amounts that it causes no harm. It has been estimated that small children (ages 2–4) ingest about 0.3 g of toothpaste every time they brush their teeth and that adults tend to ingest about 0.04 g per brushing.
(Rohrig, B. How Toxic is Toxic? ChemMatters, 2014, 32 (4), p 7)
And fluoride is not without its problems, as this brief article from a 1932 J. Chem. Educ. article shows:
Fluorine proved cause of mottled teeth
Dogs with mottled teeth, an endemic condition of the enamel produced by the presence of fluorine in drinking water, have been achieved experimentally by Dr. Margaret Cammack Smith of the home economics department at the University of Arizona.
Six months ago, Dr. Smith completed her experiments with the drinking water at St. David, Arizona, and determined that fluorine in the drinking water at that place was responsible for the existence of mottled teeth.
At first the mottled condition was only produced experimentally in white rats but now for the first time this condition has been given to the larger animals. The mottled condition has been produced after a six months' feeding experiment.-Science Service
(Fluorine proved cause of mottled teeth. J. Chem. Educ., 1932, 9 (5), p 858)
More on ways diet can help to prevent tooth decay
Diet can play a major role in preventing caries formation. These recommendations come from the article “Sugar and Dental Caries” by Riva Touger-Decker and Cor van Loveren, published in the American Journal of Clinical Nutrition in 2003:
1) eat a balanced diet rich in whole grains, fruit, and vegetables and practice good oral hygiene—particularly the use of fluoridated toothpastes—to maximize oral and systemic health and reduce caries risk.

2) eat a combination of foods to reduce the risk of caries and erosion; include dairy products with fermentable carbohydrates and other sugars and consume these foods with, instead of, between meals; add raw fruit or vegetables to meals to increase salivary flow; drink sweetened and acidic beverages with meals, including foods that can buffer the acidogenic effects.

3) rinse mouth with water, chew sugarless gum (particularly those containing sugar alcohols, which stimulates remineralization), and eat dairy product such as cheese after the consumption of fermentable carbohydrates.

4) chew sugarless gum between meals and snacks to increase salivary flow.

5) drink, rather than sip, sweetened and acidic beverages.

6) moderate eating frequency to reduce repeated exposure to sugars, other fermentable carbohydrates, and acids.

7) avoid putting an infant or child to bed with a bottle of milk, juice, or other sugar-containing beverage.

(Touger-Decker, R. and van Loveren, C. Sugar and Dental Caries. Am J Clin Nutr, 2003, 78 (suppl), 881S–92S; http://ajcn.nutrition.org/content/78/4/881S.full.pdf+html)

It has been suggested (and studies show) that chewing gum helps to prevent tooth decay. That, apparently, depends on the type of gum and how long it is chewed. It seems logical that chewing gum stimulates the flow of saliva in the mouth, and that can help to neutralize acids produced by plaque bacteria.
On the other hand, sugared gum tends to coat your teeth with sugar, which can lead to tooth decay, the build-up of plaque and the proliferation of bacteria. This is especially true if the gum is removed soon after the flavor is gone. But if gum is chewed for 10-20 minutes, some experts hold that it will then act to decrease tooth decay. One study showed that after chewing gum for 10 minutes each waking hour for two weeks, participants in the study increased their salivary flow, the pH of their saliva, and its buffering capacity, all of which would tend to neutralize some of the acid produced by mouth—the cause of tooth decay.
Sugar-free gum may actually pose more potential problems if not chewed in moderation. Sugar free gums contain hexitols, sorbitol and mannitol as sugar substitutes. The ingestion of these substances can cause diarrhea, as they are not absorbed, but instead pass into the small intestine and colon. It only takes about 10 grams of sorbitol to produce this effect in many individuals. One flight attendant, who had been experiencing abdominal pain and diarrhea for over seven years was given a wide range of diagnostic tests to no avail, but upon questioning, it was found that she had been consuming about 60 sticks of sugar-free chewing gum a day, representing about 75 grams of sorbitol. Upon ceasing to chew gum, her symptoms disappeared.
But then again, there are studies which indicate that one artificial sweetener, Xylitol (e.g. Xylifresh gum) can act to reduce tooth decay if chewed in moderation—two pieces of gum three to five times daily for at least five minutes per chewing session.
(ChemMatters Teacher’s Guide, Dec 2000, Chewing Gum)
Research is being done on other foods to see if they might have decay-preventative properties. Score one (more) for the benefits of drinking coffee!
Coffee may also protect teeth. Farah, Gazzani, and Beatriz Gloria, a chemistry professor at the Universidade Federal de Minas Gerais, Belo Horizonte, Brazil, have shown that chemicals in roasted—but not green—coffee inhibit the growth of bacteria that cause tooth decay.
The scientists found a variety of different antibacterial chemicals which killed or inhibited the growth of Streptococcus mutans, the major cause of dental decay in humans. Also, Gazzani and colleagues applied roasted coffee to hydroxyapatite, a component of tooth enamel—the hard white substance covering a tooth—and showed that small molecules present in coffee prevented S. mutans bacteria from binding to it.
(Haines, G.K. Coffee: Brain Booster to Go. ChemMatters, 2008. 26 (4), p 9)
More on fillings
A 1929 article in the Journal of Chemical Education by a professor from a dental school discusses the needs of dentists in terms of dental fillings and cements needed to hold them in place, and the then-state-of-the-art developments in dental materials, made by chemists. The requirements listed below have not changed in the interim, although the materials available (thanks to chemists) have come a long way.
Requirements of Filling Materials
The dentist continues to seek new and improved materials for filling teeth. Few of the materials now available can be regarded as perfect from his standpoint.
A list of the requirements of a perfect filling material is about as follows-:

  1. The material should be indestructible in the fluids of the mouth. It must be remembered that saliva is ordinarily somewhat acid and all cements and even some amalgams are slowly dissolved out in the mouth.

  2. The material should have adaptability to the walls of the tooth cavity, i.e., the dentist should be able to mallet or tamp it to place.

  3. It must be free from shrinkage or expansion after placing in the tooth.

  4. It must be hard enough to resist the attrition and wear of mastication.

  5. It must be strong and tough to prevent fracture or displacement by the stresses of mastication.

  6. The material should have a good color and appearance. If possible it should be available in shades to match the color of tooth structure.

  7. It is highly desirable that the filling material should be a non-conductor of thermal changes-heat and cold.

  8. It must be remembered that filling material is placed in a living tooth and it must have no toxic effect on the pulp or "nerve."

  9. The material should be easy to manipulate, not only readily prepared or mixed, but, if plastic, it should harden promptly when inserted in the tooth.

No material available today for filling teeth possesses all of these desirable qualities. Some materials, like the gold inlay and silver amalgam, possess enough of these properties to make them satisfactory: some, like the cements, are used because nothing better is available.

(Brightfield, L. The Dentist’s Problem—Satisfactory Material for Restoring Teeth. J. Chem. Educ., 1929, 6 (2), p 308)
The author also discusses at length the state of the art with regard to dental cements used to hold the fillings in place. He concludes his article by saying that
It is likely that some of these dental requirements will be difficult to meet.
But the dentist is confident that the chemist will continue to study dental cements and eventually give him a formula that will answer most if not all his requirements. The dentist's greatest problem has always been that of finding satisfactory materials for restoring teeth and with the aid of the chemist his problem is on the way to satisfactory conclusion.
(Ibid; p 313)
Here is a similar list from 1999, 70 years later:
Ideally, a dental restorative material should be perfectly compatible with the oral environment and should fulfill the criteria set out below:

easily mixed and placed as an unset paste

short working and setting times

rapid buildup in mechanical properties on setting

match of thermal and expansion properties with the tooth

high resistance to erosion and degradation by oral fluids/saliva, brushing, and flossing

biologically inert or bioactive

achieves a hermetic seal with the surrounding tooth tissue

color and translucency to match the tooth

high strength (tensile and compressive)

(Nicholson, J.; Anstice,H. Chemistry Everyday for Everyone: The Chemistry of Modern Dental Filling Materials. J. Chem. Educ., 1999, 76 (11), pp 1497–1501; abstract available online at http://pubs.acs.org/doi/abs/10.1021/ed076p1497; article available to subscribers only at this same URL)
Comparing the two lists, we have:

Desirable Physical Properties of Dental Fillings

Brightfield, 1929

Anstice, 1999

Indestructible in the mouth

High resistance to erosion by saliva, etc.

Adaptable to walls of tooth, tampable, malleable

Achieves hermetic seal with surrounding tooth tissue

Free from shrinkage

Match expansion properties of tooth

Hard – resistant to wear and chewing

High strength

Strong and tough to prevent fracture

High tensile and compressive strength

Good color and appearance

Color and translucency to match tooth

Non-conductor of thermal heat or cold

Match of thermal properties of tooth

Non-toxic to root or nerve

Biologically inert or bioactive

Easy to manipulate; harden quickly in place

Easily mixed–short working and setting time–rapid buildup in mechanical properties on setting


The list from 1999 would be relatively unchanged, even to present times. As you can see, the two lists have not changed in 70-plus years, although the means to achieving these properties has changed significantly, with the invention of the mercury amalgam and, now, composite polymeric materials.

More on dental amalgams & mercury
There is no question that there is controversy surrounding the use of mercury amalgam fillings. And yet, almost all professional dental organizations worldwide say that, at the present time, they find them to be safe to use. Here is some background information about amalgam fillings.
Dental amalgam is a dental filling material used to fill cavities caused by tooth decay. It has been used for more than 150 years in hundreds of millions of patients around the world.
Dental amalgam is a mixture of metals, consisting of liquid (elemental) mercury and a powdered alloy composed of silver, tin, and copper. Approximately 50% of dental amalgam is elemental mercury by weight. The chemical properties of elemental mercury allow it to react with and bind together the silver/copper/tin alloy particles to form an amalgam.
Dental amalgam fillings are also known as “silver fillings” because of their silver-like appearance. Despite the name, "silver fillings" do contain elemental mercury.image of a capsule containing liquid mercury and amalgam putty.
When placing dental amalgam, the dentist first drills the tooth to remove the decay and then shapes the tooth cavity for placement of the amalgam filling. Next, under appropriate safety conditions, the dentist mixes the powdered alloy with the liquid mercury to form an amalgam putty. (These components are provided to the dentist in a capsule as shown in the graphic.) This softened amalgam putty is placed and shaped in the prepared cavity, where it rapidly hardens into a solid filling.
(U.S. Food and Drug Administration (FDA): http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/DentalProducts/DentalAmalgam/ucm171094.htm)
Here is another description of the amalgam capsule, from a previous (1996) ChemMatters article.
A dental amalgam capsule contains two chambers that keep the ingredients separate. When the ingredients are mixed, they form a paste that begins to harden into solid metal in just a few minutes. For this reason, the final mixing must be done in the dentist's office, just before the drilled cavity is filled. The capsule is placed in a machine that shakes it back and forth vigorously. The vibration ruptures the barrier between the two compartments and thoroughly mixes with [sic] mercury with the powdered metals.
(Graham, T. Nightmare on White Street. ChemMatters, 1996, 14 (4), pp 9–11)
People worried about mercury in their mouth/body often ask if they should have amalgam fillings removed and replaced with composite fillings. According to the FDA, “If your fillings are in good condition and there is no decay beneath the filling, FDA does not recommend that you have your amalgam fillings removed or replaced. Removing sound amalgam fillings results in unnecessary loss of healthy tooth structure, and exposes you to additional mercury vapor released during the removal process.” (U.S. Food and Drug Administration: http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/DentalProducts/DentalAmalgam/ucm171094.htm)
The following quote from a 2009 white paper updates the FDA position on the use of mercury amalgams: “It is concluded that there is insufficient evidence to support an association between exposure to mercury from dental amalgams and adverse health effects in humans, including sensitive subpopulations.” (http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/DentalProducts/DentalAmalgam/ucm171117.htm#6)
Here is a list, from the Department of Health and Human Services, Public Health Service, of advantages and disadvantages of mercury amalgam in dental fillings:

Table 1.
Comparison of the Advantages and Disadvantages of
Dental Amalgam as a Restorative Material



  • Durable

  • Least technique sensitive of all restorative materials

  • Applicable to a broad range of clinical situations

  • Newer formulations have greater long-term resistance to surface corrosion

  • Good long-term clinical performance

  • Ease of manipulation by dentist

  • Minimal placement time compared to other materials

  • Initially, corrosion products seal the tooth-restoration interface and prevent bacterial leakage

  • One appointment placement (direct material)

  • Long lasting if placed under ideal conditions

  • Often can be repaired

  • Economical

  • Some destruction of sound tooth tissue

  • Poor esthetic qualities

  • Long-term corrosion at tooth-restoration interface may result in "ditching" leading to replacement

  • Galvanic response potential exists

  • Local allergic potential

  • Concern about possible mercury toxicity

  • Marginal breakdown


Lest the reader think that “any old” amalgam will do, dentists early on discovered the need for exact amounts of the components and precise measurement of the mixture, in order to ensure a tight fit and good seal when the amalgam is placed in the newly drilled cavity, and thereafter. The hardened metal alloy used in fillings has a coefficient of expansion very different from that of tooth dentin and enamel. Thus it will change volume within the tooth when exposed to temperature changes. When the filling’s temperature increases, with hot food or drink, it expands, more than the tooth; when the temperature drops with cold food or drink, it shrinks, again, more than the tooth.
The tooth is strong enough to withstand the expansion component, but when the filling shrinks, it could possibly open the sides of the cavity, between the tooth material and the amalgam, exposing it to bacterial infestation and infection. Thus it must be “pre-expanded”, so that future temperature-related shrinkage does not result in re-opening the cavity.
Chemists long ago experimented with varying amounts of the metals in the amalgam mix, until they discovered the best mix for this purpose, per the dental association’s parameters of 3–13 micrometers per centimeter of amalgam. The carefully measured mix of ingredients in the amalgam increases its volume slightly as the alloy is hardening, and thus it somewhat forcefully seals shut the cavity. This slight expansion of the alloy as it hardens within the cavity results in a slight compression of the material within the tooth, allowing for slight shrinkage when cold food or drink lowers its temperature, but not enough to open the cavity.

(Philips, R. Dental Amalgam: A Reaction Involving Measurement of Minute Dimensional Change. J. Chem. Educ., 1945, 22 (3), p 117; first page at http://pubs.acs.org/doi/abs/10.1021/ed022p117, entire article available to subscribers only at same URL)

More on composite fillings
Composite fillings are made primarily of polymeric materials, which exist in the composite resin material as oligomers, short-chain organic molecules, in a matrix. Some of the most used materials include:
… a bisphenol A-glycidyl methacrylate (BISGMA) or urethane dimethacrylate (UDMA), and an inorganic filler such as silicon dioxide (silica). Compositions vary widely, with proprietary mixes of resins forming the matrix, as well as engineered filler glasses and glass ceramics. The filler gives the composite wear resistance and translucency. A coupling agent such as silane is used to enhance the bond between these two components. An initiator package (such as: camphorquinone (CQ), phenylpropanedione (PPD) or lucirin (TPO)) begins the polymerization reaction of the resins when external energy (light/heat, etc.) is applied. A catalyst package can control its speed.
Advantages of composite fillings include:

  • Esthetics—composite fillings are typically white, or tooth-colored, rather than the silver or black of amalgam fillings

  • Less tooth damage—more healthy tooth material must be removed for amalgam fillings in order to ensure a tight lock-and-key fit for the amalgam; in composites, the filling is glued (bonded) in place, so less tooth is removed

  • Bonding to tooth—amalgams are mechanically held in place, but composites are actually bonded to the tooth material chemically, ensuring a stronger bond and, hence a stronger tooth

  • Possible prevention of tooth removal—if large portions of a tooth are decayed, it may be too much to allow an amalgam filling, but composites may still be used to preserve and strengthen the tooth

  • Versatility—composites can be used to repair cracked or chipped teeth, not possible with amalgams

  • Maintainability—minor damage to a composite filling may be repaired using additional composite material laid down over top of the original filling; amalgam filling damage would require removal of the old filling and replacing it with an entirely new filling

  • Environment—no mercury in the body, no mercury in the dentists’ offices, no mercury in wastewater

But of course nothing has only advantages; here are some disadvantages of composite fillings:

  • Durability—composites may not last as long as amalgams, especially in large cavities, and where they bear the brunt of chewing

  • Shrinkage—composites shrink a bit more than amalgams, leaving the areas around the filling, next to tooth material, subject to microleakage which can lead to secondary caries; new formulations of composites reduce the shrinkage factor

  • Chipping—exposed edges of composite fillings can chip off

  • In dentist’s office—placing composites requires more training, skill and talent than needed for amalgams; placing composites takes more time to do than for amalgams; a completely dry environment is needed to place composites, not so for amalgams

  • Cost—because they take longer to do, composites are more costly to place than amalgams, so dentists may charge more for composites; because composites are more costly, insurance companies frequently do not cover entire cost of composite filling

Composites were developed as an improvement over amalgam fillings, which they seem to be in most cases. And they certainly pose less of an environmental and health hazard than mercury amalgam fillings. Nevertheless, their safety has been questioned due to the fact that some composites can emit bisphenol-A (BPA), a known endocrine-disruptor, when they are placed in a filling. The emission seems to last only a short while (<1 hour) after they are placed, so exposure is not chronic, as is the case with mercury.

This emission could be from leftover BPA used in the monomer/oligomer resin that was not polymerized into the matrix, or as the result of the degradation of some of the resin after polymerization. Some studies have shown that the amount of BPA released is insignificant.
More on possible new developments in preventing/treating tooth decay
Here’s a possible new treatment to accelerate remineralization of a dental cavity. According to Dr. Margaret Culotta-Norton, a dentist in Washington, D.C., and former president of the D.C. Dental Society, this would eliminate the need for dental fillings which, she says, generally require repair or replacement, often several times over a lifetime.
Culotta-Norton said that a new treatment for cavities may be on the horizon. A process called electrically accelerated and enhanced remineralization (EAER) is being developed in London. She explained that this process "accelerates the natural movement of calcium and phosphorous minerals into the cavity to repair it. This process would eliminate drills and injections. It emits tiny electrical currents into the tooth to push the minerals into the repair site. It encourages the tooth to repair itself." According to The Guardian, this new process could be available by 2017.
On a similar note, news (as of May 2012) from the University of Maryland School of Dentistry tells us about a nanomaterial that acts as an antibacterial agent is being added to existing—and new—composites to control the growth of bacteria in a cavity that has been prepped for a filling. It is very difficult, even impossible, to remove 100% of the decay in a cavity. Leftover bacteria are able to continue growing and reproducing—and producing acids that will continue to eat away at the enamel and dentin in the cavity—even after the filling has been placed, increasing the likelihood that the filling will eventually fail.
The new material is composed of existing composites and quaternary ammonium and silver nanoparticles, at a high pH (which helps to neutralize acid produced by bacteria). Another key feature is the addition of amorphous calcium phosphate, which helps to remineralize the tooth. This combined material is used primarily in the primer used by dentists to prepare the drilled-out cavity, and in the adhesive used to coat the cavity surface to help the composite filling stick to the remaining surface of the tooth. (http://www.sciencedaily.com/releases/2012/05/120501182830.htm)
This study shows that adding hydroxyapatite (HAP) nanofibers to existing composite resins effectively reinforces the composites and significantly improves biaxial flexural strength of the composites. (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3422879/)
And here’s a somewhat novel approach, to genetically engineer a non-stick strain of bacteria that will adhere to tooth enamel so that S. mutans can’t!
No cavities!
Through genetic engineering, strains of harmless bacteria have been altered to carry desirable traits into plants, making them disease and insect resistant. Other strains have been altered to be tiny factories for otherwise very expensive proteins, insulin and human growth hormone, to name just two. Soon a new strain of Salmonella may be used as an anticavity vaccine, and tooth decay may become only a bad memory The bacteria Streptococcus mutans form plaque on teeth, where they convert sugars in food to acids. Once these acids dissolve tooth enamel, teeth decay rapidly. Dr. Roy Curtiss of Washington University in Missouri discovered how Streptococcus mutans bacteria stick to teeth. One of the proteins on the surface of the bacteria, called "SpaA," attaches to tooth enamel first, then other surface proteins convert the loose hold to a much tighter one.
The human body forms antibodies against foreign proteins, and should do so against SpaA as well, if enough is present. If surface SpaA were attacked, the bacteria couldn't attach themselves to teeth! Animal experiments on rats and monkeys have been promising, but because Streptococcal bacteria are implicated in heart and kidney damage, great caution is needed.
The antibody response cannot be created directly from the bacteria. SpaA is believed safe enough itself, so a harmless strain of Salmonella bacteria was genetically altered to produce SpaA. When injected in a vaccine, the Salmonella attach themselves to lymph nodes of the small intestine and go to work, producing lots of SpaA, enough to trigger the production of lymphocytes (produce antibodies). They migrate through the lymph system to the salivary glands, where the lymphocytes protect against new SpaA and give a permanent protection against Streptococcus mutans. Even if an anti-decay vaccine becomes a reality, you will still have to brush and floss regularly. No one wants to kiss someone with rotten gums and bad breath.
(ChemComments: No Cavities! ChemMatters, 1990, 8 (2), p 16)
I have to wonder what ever happened to this idea, since 25 years have elapsed. …

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