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



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Background Information


(teacher information)

To begin this discussion, it might be nice to think about the teeth as part of a larger structure. This excerpt is from a 2000 ChemMatters article:

The mouth is like the entrance to a deep cave. Inside are minerals, a steady trickle of water, and living creatures! Teeth line the upper and lower jaws like stony stalactites and stalagmites composed of protein (collagen) and a hard smooth mineral called hydroxyapatite, Ca5(PO4)3OH. Along the inner walls of the mouth are glands that secrete saliva, a watery solution that flows into the mouth at 1 to 3 mL per minute at mealtimes but slows to barely a trickle during sleep.
Inhabiting the mouth are millions of living bacteria residing on the tongue, in the soft tissue of the gums, and inside the cracks and crevices of our teeth. Many of the metabolic wastes of these bacteria are both corrosive and sticky with a pH low enough to cause harm to teeth and gums. Dissolved in saliva is bicarbonate (HCO3). Bicarbonate acts as a buffer to keep the watery solution at a fairly constant pH by balancing the relative amounts of hydrogen ions (H+) and hydroxide ions (OH) in solution. A healthy pH for the mouth environment is a nearly neutral 6.8.
Saliva is saturated with enzymes, the specialized proteins that act as organic catalysts for a variety of chemical reactions in the body. Alpha amylase is a digestive enzyme in saliva that catalyzes the breakdown of starch. Starch—a natural polymer consisting of thousands of tiny sugar molecules linked together like boxcars on a train—is rapidly uncoupled by amylase to release these sugars in the mouth.
Sugar is food. We—and the bacteria that we harbor—obtain life-sustaining energy from the breakdown of this hydrocarbon fuel. Unfortunately for us, bacteria convert some of this sugar into harmful acids. Saliva acts to dilute and neutralize some of this acid, but bacteria living in teeth fissures or crevices may be protected from this cleansing.
(McClure, M. The Straight Story on Braces. ChemMatters, 2000, 18 (1), pp 7–8)
Now that we’ve got the “big picture”, we can proceed with some of the details of tooth decay.
More on the structure of the tooth
The tooth consists of three areas: the crown, the neck and the root. (The neck is the area where the crown meets the root, missing from the diagram below.) The internal structure of the tooth is composed of four parts: enamel, dentin, cementum and pulp.
(https://en.wikiversity.org/wiki/Wikiversity_Journal_of_Medicine/Blausen_gallery_2014#/media/File:Blausen_0863_ToothAnatomy_02.png)

The enamel is located on the surface of the crown. The dentin, which lies just under the enamel, extends through the crown, neck and root, as does the pulp. Cementum surrounds the root. What follows is a description of each of these parts.


Enamel
Enamel is the very hard outer surface of the tooth. It is the surface we see when we look at teeth. It appears white, but it is really translucent. The dentin (see below) that shows through the enamel gives the tooth its color.
The role of enamel is to provide the rigid surface needed for mastication—grinding and crushing food by chewing—and to protect the rest of the underlying layers of the tooth from decay. It owes its rigidity to its structure of hydroxyapatite, a crystalline calcium phosphate compound. Although it is a hard substance, it is susceptible to decay through erosion due to exposure to acid. It also serves to insulate the nerves in the tooth from exposure to extremes of hot and cold thus preventing discomfort or pain.
Enamel is also subject to cracking or chipping when exposed to stress, leading to one’s feeling pain, especially when eating hot or cold or sugary foods.
(https://www.humana.com/learning-center/health-and-wellbeing/healthy-living/tooth-enamel)
And, lest we oversimplify the enamel in tooth structure, this short paragraph from a ChemMatters Teacher’s Guide seeks to set us straight.
The structure and composition of a human tooth is perhaps somewhat more complex than the relatively basic structure and composition presented …. The outer enamel is indeed the hardest material found in the human body, as it is for any mammal that has teeth. It is highly mineralized, but not entirely made of calcium phosphate. It consists of about 95-98% inorganic material by mass. About 90-92% of this inorganic matter is a slightly modified form of calcium phosphate called hydroxyapatite. The formula for hydroxyapatite is Ca5(PO4)3OH. There are also trace amounts of other minerals. The remainder of the enamel consists of about 1% protein and 4% water by mass. The proteins that are contained in tooth enamel are not found anywhere else in the human body. These proteins are called enamelins and amelogenins.
(ChemMatters Teacher’s Guide, December 2003, p 29)
Dentin
Dentin, part calcified tissue (hydroxyapatite crystallites), part organic material (mainly collagen), and part fluid (mainly water), surrounds the pulp cavity, just under the enamel. It serves several purposes: to absorb the impact of mastication on the tooth enamel, to protect the pulp from infection from the outside (from bacteria in the bacteria-infested mouth), and to provide toughness to the tooth structure, preventing or, at least, minimizing tooth fractures. (Note that enamel, even though it is very hard, is also rather brittle, so it can fracture rather easily.)
Dentin contains dentinal tubules, which are permeable, that radiate outward from the center to the enamel. Mineral buildup surrounds these tubules. Their permeability allows for transfer of the sensations of heat and cold to nerves in the pulp which can, in turn, become sensations of pain. The dentinal tubules also help to prevent tooth fractures by absorbing some of the stress that might normally propagate through and fracture the enamel, forming microfractures within the tubules that prevent a major crack from propagating through the brittle enamel.
It has been noted that older adults seem to be more susceptible to tooth fracturing than younger people. This has been researched and is presently believed to be caused by subtle changes in the behavior of dentin in older teeth, resulting in its becoming more brittle with age. This paper describes current (2008) research: https://str.llnl.gov/str/JanFeb08/pdfs/01.08.3.pdf.
Cementum
Cementum is the surface layer of the tooth root. It is calcified material that covers the root of the tooth. Slightly softer than dentin, it consists of 45–50% inorganic material (hydroxyapatite) and 50–55% organic matter and water, by weight. Collagen and proteoglycans comprise the majority of the organic matter.
Cementum is the part of the periodontium that attaches the tooth to the alveolar bone. Because some of the cementoblasts, the cells that actually excrete the cementum, are entrapped within the cementum, becoming cementocytes, cementum is able to repair itself to a limited extent.
Pulp
Dental pulp serves two main roles. First, it produces odontoblasts, cells that can form dentin. The dentin surrounds and protects the pulp. The second role is to supply nutrients to, and remove waste from, the pulp via blood vessels contained in the pulp cavity. Other functions include signaling the brain (with pain) when trauma, temperature extremes, pressure or tooth decay has reached the dentin or pulp, areas containing nerves, and forming secondary dentin to help protect the pulp.
Dental pulp fills with an increased amount of collagen fibers with age, resulting in the decrease in the ability of the pulp to regenerate. This causes the recession of the pulp cavity, possibly due to an increase of secondary dentin within the pulp cavity, which results in the reduction in sensitivity in older teeth. Thus older adults may not need local anesthesia when undergoing dental restorations.
The web source for the diagram at the beginning of this section also contains a 3-D diagram of the tooth and a brief (0:50), narrated video clip on the anatomy of a tooth. (http://blausen.com/?Topic=2106) (Source of diagram above and video clip: Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. - Own work)
More on the chemical structure of tooth enamel
Tooth enamel, as mentioned in the article, is the hardest substance in the human body. The enamel is composed of 96% minerals, primarily hydroxyapatite; the remaining 4% of the enamel is primarily water and organic material. Apatite, the primary constituent of tooth enamel, has a hardness of 5 on the 1–10 Mohs scale of mineral hardness.
©C. Robinson Oral Biology

The structure of calcium hydroxyapatite.

(https://www.academia.edu/1732481/Dental_Enamel_Chemistry)

The central oxygen atom in the unit cell diagram at right is part of one of the


–OH groups in the hydroxyapatite formula, Ca10(PO4)6(OH)2 [the dimer of Ca5(PO4)3OH]. In fluorapatite, the fluorine atom replaces that oxygen atom (part of
–OH) in the center of the hexagonal unit cell.
The following passage describes the structure of hydroxyapatite.
The term "apatite" applies to a group of compounds (not only at calcium phosphates) with a general formula in the form M10(XO4)6Z2, where M2+ is a metal and species XO4 3- and Z- are anions. The particular name of each apatite depends on the elements or radicals M, X and Z. In these terms, hydroxyapatite (HAp) has the molecular structure of apatite, where M is calcium (Ca2+), X is phosphorus (P5+) and Z is the hydroxyl radical (OH-). This is known as stoichiometric hydroxyapatite and its atomic ratio Ca/P is 1.67. Its chemical formula is Ca10(PO4)6(OH)2, with 39% by weight of Ca, 18.5% P and 3.38% of OH.
Hydroxyapatite crystallizes in a hexagonal system … Figure 1 shows the unit cell of hydroxyapatite.

Figure 1. Crystalline structure of hydroxyapatite.

HAp structure is formed by a tetrahedral arrangement of phosphate (PO43-), which constitute the "skeleton" of the unit cell. Two of the oxygens are aligned with the c axis and the other two are in a horizontal plane. Within the unit cell, phosphates are divided into two layers, with heights of 1/4 and 3/4, respectively, resulting in the formation of two types of channels along the c axis, denoted by A and B.


The walls of channels A type are occupied by oxygen atoms of phosphate group and calcium ions, called calcium ions type II [Ca (II)], consisting of two equilateral triangles rotated 60 degrees relative to each other, at the heights of 1/4 and 3/4, respectively. Type B channels are occupied by other ions of calcium, called calcium ions type I [Ca (I)]. In each cell there are two such channels, each of which contains two calcium ions at heights 0 and 1/2. In the stoichiometric HAp, the centers of the channels type A are occupied by OH radicals, with alternating orientations. …
Despite being taken to the stoichiometric hydroxyapatite as a model, it is noteworthy that hydroxyapatites produced biologically are much more complicated, they are not stoichiometric, have an atomic ratio Ca/P <1.67 and does not contain only ions and radicals of the HAp but also traces of CO3, Mg, Na, F and Cl. These amounts vary according at the specific type of tissue, which is related to the properties and bioactivity of it.
One aspect that is important to note is that, the closer the value of Ca/P to 1.67, the greater the stability of the material inside the human body as they tend to be inert, and on the other hand, if this value decreases (deficient HAp), the better the bioactivity.

Another aspect we must consider is the degree of crystallinity. It has been observed that the crystallinity in the tissues for the tooth enamel is very high, while in the cases corresponding to dentin and bone, it is very poor. This means that the reactivity depends on the degree of crystallinity, since the reactivity in dentin and bone is higher than in tooth enamel.


(Eric M. Rivera-Muñoz (2011). Hydroxyapatite-Based Materials: Synthesis and Characterization, Biomedical Engineering - Frontiers and Challenges, Prof. Reza Fazel (Ed.), ISBN: 978-953-307-309-5, InTech, DOI: 10.5772/19123; http://www.intechopen.com/books/biomedical-engineering-frontiers-and-challenges/hydroxyapatite-based-materials-synthesis-and-characterization)
More on tooth decay vs. tooth erosion
There seems to be general agreement that two distinct processes occur involving adverse effects on teeth, tooth erosion and tooth decay.
Tooth erosion
Microscopic view of erosion on tooth enamel surface.

(http://www.webmd.com/oral-health/healthy-teeth-14/slideshow-enamel-erosion)

Tooth erosion occurs primarily due to acids you ingest from outside sources, such as sodas and citrus fruit drinks. These provide acid directly to the tooth, which increases the rate of demineralization of the tooth enamel, which eventually leads to erosion of tooth surfaces and may or may not produce individual caries (cavities).

(https://en.wikipedia.org/wiki/Acid_erosion)
Frequency and duration of exposure of teeth to these drinks is viewed as more important factors than total intake. These should be drunk, not sipped. Many dentists even recommend using straws to drink these, as then the liquid does not come in direct contact with teeth. Enamel corrosion can even occur in babies if they are allowed to drink fruit juices from a bottle over long periods as a way of quieting them down at bedtime.
The following table summarizes the types of acids found in various types of drinks.


Type of Drink

Type of Acid in Drink

Natural or Added

Purpose/Use

Soda/Pop

Carbonic

Phosphoric

Citric (perhaps)


Added

Added


Added

Fizz, “bite”

Tartness, preservative

Fruity taste


Fruit Juices

Malic

Citric


Ascorbic (vitamin C)

Tartaric


Natural

Natural/Added

Added

Added


In most fruits, tartness

Citrus flavor

Preservative

Acidity, tartness



Juice Drinks

Citric

Ascorbic


Fumaric

Natural/Added

Added


Added

Citrus flavor

Preservative

Tartness


Sports/Energy Drinks

Carbonic

Citric


Added

Added


Fizz, “bite”

Citrus flavor



Wines

Tartaric

Malic


Lactic

Citric


Natural

Natural


Added

Added


Stability, acidity, tart taste

Tartness, apple flavor

“Milky” flavors

Boosts overall acidity



Beers

Carbonic

Natural

Fizz, “bite”

The phosphoric acid is corrosive, but actually the acid concentration in soda pop is lower than that in orange juice or lemonade. Try submerging identical strips of magnesium (or iron staples) in each of these beverages overnight. Which beverage dissolves more metal? Which dissolves the metal fastest?


Fruit juices and drinks are also tart, but they don't use phosphoric acid as a flavor additive. Phosphoric acid would cause many ions present in fruit juices to settle out as insoluble phosphates. These beverages get their tang from citric acid, a substance found in oranges, limes, lemons and grapefruits. Malic acid, found in apples and cherries, is added to many fruit juices. Fumaric acid is used in noncarbonated soft drinks, and tartaric acid gives grape-flavored candies a subtle sour flavor. All of these substances impart only tartness, without overpowering other flavors present.
(http://antoine.frostburg.edu/chem/senese/101/consumer/faq/why-phosphoric-acid-in-soda-pop.shtml)
Tartaric acid isn't added to grape-flavored beverages because of the low solubility of some of its salts:
"... tartaric acid gives a very true flavor, but Mother Nature does not intend for tartrates to stay in solution long. When KH-tartrate precipitates out of a juice, looking very much like glass or metal shavings, and the consumer passes their bottle of juice to the FDA, one really does not care about "true" flavor. We in the juice industry usually use malic or a malic citric blend."
(http://antoine.frostburg.edu/chem/senese/101/consumer/faq/why-phosphoric-acid-in-soda-pop.shtml)
Chemists know that it’s acid strength, not just the amount of acid, that really matters. That’s why colas are more likely to cause dental erosion than other sodas; colas contain phosphoric acid, with a higher acid dissociation constant (see table, below) than any of the other acids listed in the drinks from the above table. That means that phosphoric acid provides more H+ ions in solution than other acids. These ions then interact with enamel hydroxyapatite, resulting in the formation of Ca2+ and PO43– ions dissolved from the tooth surface. Unless the saliva in the mouth quickly raises the pH and replenishes the lost calcium and phosphate ions, the tooth enamel surface will remain thinner where those ions were removed by the acid, subject to further degradation with the next cola drink.


Acid

1st Ka

2nd Ka

3rd Ka

Phosphoric acid

7.5 x 10–3

6.2 x 10–8

4.8 x 10–13

Fumaric acid

9.3 x 10–4

2.9 x 10–5

---

Tartaric acid

9.2 x 10–4

4.3 x 10–5

---

Citric acid

8.4 x 10–4

1.8 x 10–5

4.0 x 10–6

Malic acid

3.5 x 10–4

8.0 x 10–6

---

Lactic acid

1.4 x 10–4

---

---

Ascorbic acid

7.9 x 10–5

1.6 x 10–12

---

Carbonic acid

4.3 x 10–7

4.7 x 10–11

---

(Table of Kas gathered from numerous sources)


Notice that the Ka for carbonic acid is the smallest of any of the first dissociation constants for the acids in the above table. This substantiates the notion that it’s not the carbonic acid in sodas that really causes the problems with dental erosion but, rather, phosphoric acid. This can be shown by testing the pH of a freshly-opened soda and one that has been allowed to go “flat”. Carbon dioxide has escaped from the flat soda, upsetting the CO2 – H2CO3 equilibrium, thereby removing most/all carbonic acid from the drink (Le Châtelier’s principle). Yet both sodas will have approximately the same pH, showing that carbonic acid contributed very little or nothing to the acidity of the drink.
While brushing teeth is a good way to minimize tooth erosion, care must be taken as to when brushing is done.
Tooth brushing is a way to keep a good oral hygiene. Hard tissue loss after erosion and tooth brushing is significantly greater than erosion alone … However, after intra-oral periods of 30 and 60 min, wear was not significantly higher in tooth brushing than in unbrushed controls. It is concluded that keeping tooth unbrushed for at least 30 min after an erosive attack is necessary for protecting dentin …
(http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2676420/)
You may have heard this myth circulating, possibly on the Internet. “Soda is so acidic it can dissolve a tooth overnight.” This is not quite true (at all).
This myth got its start from a nutritionist who made the claim in the 1950s. Sodas contain acids, such as phosphoric, citric, and carbonic acid. But their concentrations are lower in soda than in natural drinks, such as orange or cranberry juice. When left in soda, a tooth will not completely dissolve overnight, or even over a few days. Also, when we drink soda, we don’t tend to hold it in our mouths for long periods of time, and the saliva in our mouths helps protect the enamel.
But this does not mean that soda is harmless to teeth. High-sugar drinks can contribute to tooth decay, and acidic drinks can erode tooth enamel over time. The reason is that although enamel is hard, the substance that makes up most of it, hydroxyapatite [Ca5(PO4)3OH], is in equilibrium with its dissolved form, like any ionic solid in the presence of water. At equilibrium, most of hydroxyapatite is in solid form:
Ca5(PO4)3OH (s) ⇌ 5 Ca2+ (aq) + 3 PO43– (aq) + OH (aq)
But when an acid is added, its free hydrogen ions (H+) neutralize some of the hydroxide ions (OH), as follows:
H+ + OH ➞ H2O
This shifts the hydroxyapatite equilibrium reaction to the right to replace the hydroxide ions removed by the acid, causing more hydroxyapatite to dissolve, thus eroding the tooth enamel.
(Tinnesand, M. Open for Discussion: A Healthy Dose of Skepticism; Soda is so acidic it can dissolve a tooth overnight. ChemMatters, 2015, 33 (1), p 4)
Here is another reference to this same myth: http://io9.com/5903310/the-scientific-myth-that-soda-will-dissolve-your-teeth.

(http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2997506/?tool=pubmed)

Before we leave the topic of soda, we should look at its pH. The table below was taken from this 2010 report: “Pop-Cola Acids and Tooth Erosion: An In Vivo, In Vitro, Electron-Microscopic and Clinical Report.” The report, published in the International Journal of Dentistry, provides information on the pH of various colas. Note that all values are in the 2.5–3.5 range, well below the pH of 5.5, which is the pH at which (or below which) tooth enamel is eroded by an acid. Thus all of these colas (and other sodas as well) will cause enamel erosion; indeed, that is the conclusion of the report, as well.


And, as the title suggests, the study tested cola’s effects on teeth in vivo, within the mouth of living test subjects, and in vitro, in laboratory settings. And microscopic pictures of the teeth studied in vitro, taken by the researchers show definite evidence of erosion by the colas. An interesting note: their study groups were divided into those with teeth (average age, 22) and those without teeth (average 52), ostensibly those with dentures. The tests done on those without teeth (and those with teeth) consisted of determining levels of calcium and phosphate in the mouth after swishing with the various colas (and with water as a control).

(http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2997506/?tool=pubmed)


OK, so now we know it’s best to avoid sodas to avoid cavities. But sodas and other acidic drinks, known as extrinsic sources of acids (taken in from outside the body), aren’t the only way for tooth enamel to be exposed to acidic environments. Intrinsic sources (as the name implies) also may account for further exposure.
(http://www.dentalcare.com/en-US/dental-education/continuing-education/ce301/ce301.aspx?ModuleName=coursecontent&PartID=8&SectionID=2)
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