6.2: Tooth Decay

Mouth Bacteria

Tooth decay is a preventable disease caused by mouth bacteria that feast on sugar and excrete an acid waste. When the acid erodes through the enamel, bacteria enter (infect) the tooth, causing decay.

The mouth is a moist and warm habitat for bacteria, and we feed them when we feed ourselves. Bacteria are a key factor in tooth decay. Bubble-boy David from Houston (see Chap. 10) didn’t get tooth decay because he lived germ-free in a plastic bubble because of his immunodeficiency. The same is true of germ-free animals given sugar-rich diets. In this sense, tooth decay is an infectious disease—caused by bacteria.

It’s usually the mother who infects her child during the normal, intimate mother-child relationship. A pregnant woman with decayed teeth or poor dental hygiene should get her teeth filled and begin good dental hygiene. Otherwise, she’ll more easily pass decay-causing bacteria to her child through kissing, tasting the child’s food, etc.

Colonies of bacteria form in soft plaques that cling to the teeth. The bacteria feed on sugar that comes their way. Sucrose (table sugar) promotes tooth decay the most. Starch can also be a source of sugar for the mouth bacteria because saliva has amylase, the enzyme that can partially break down starch to sugar.

The bacteria break down the sugars and make lactic acid as a waste product. The acid dissolves minerals in the enamel underneath, and begins the process of decay. The initial area of mineral loss shows as a white spot on the tooth. More mineral loss causes erosion that can penetrate the enamel, allowing bacteria to infect the dentin and then the pulp (Figure 14.2). You’d feel this as a toothache. Of course, it’s best to have your dentist remove the decay and fill the cavity long before the infection progresses this far.

Unchecked, the infection can kill the pulp and cause an abscess at the tip of the root. A dead pulp means a dead tooth. Rather than pull the tooth, the dentist usually does a “root canal” by cleaning out the debris all the way to the root tips, and allowing the abscess to drain. The inside of the tooth is then sterilized and filled.

Saliva

Saliva helps prevent tooth decay. It’s slightly basic, has antibacterial agents, dilutes the acid made by bacteria, has proteins that neutralize the acid, and continually rinses the mouth of debris. Saliva’s neutralizing and diluting action doesn’t reach the acid made at the interface of dental plaque and tooth, however. Conditions or medication that lessen saliva production raise the risk of dental decay.

Saliva is rich in calcium and phosphorus and can remineralize enamel—repair some of the mineral loss. Under everyday conditions, this goes on constantly. When we eat, the bacteria eat too, and make acids that demineralize enamel. Between meals, the bacteria don’t have much to eat, allowing the plaque and saliva to become slightly basic, setting the stage for remineralization.

Remineralization is possible only at the earliest stage of mineral loss. In other words, mineral loss at a very early stage is reversible, and a cavity can be avoided. Once you get a cavity, it won’t repair itself. This is why constant snacking promotes decay, and why cleaning the teeth soon after eating helps prevent it. The time available for mineral loss versus the time available for restoring the minerals can make the difference between healthy and decayed teeth. Sucking on hard candy throughout the day gives bacteria a steady source of sugar with which to make acid—too much time for mineral loss and not enough time for restoration.

A striking example is “bottle mouth”—the rampant tooth decay (sometimes all the way to the gum line) seen in children who go to bed with a bottle of milk (milk has the sugar lactose). The child falls asleep sucking on the bottle, leaving a pool of milk in the mouth, then sucks the bottle intermittently throughout the night. A bottle of fruit juice or sugared water is, of course, just as bad. Children shouldn’t get in the habit of going to sleep with a bottle. A pacifier can be used instead.

There are other sources of acid besides mouth bacteria. Continually chewing vitamin C (ascorbic acid) tablets or sucking lemons acidifies the saliva for long periods of time. Frequent vomiting also promotes tooth decay (stomach fluid is very acid). In bulimia,* the dentist is often the first to suspect the disorder because repeated vomiting can cause irritation of the oral tissues and severe erosion and decay of the teeth.

A popular classroom demonstration is to dissolve a tooth in a glass of Coca-Cola, to show how sugar “rots the teeth.” Children are impressed but, actually, it’s the long-term acidity—not the sugar nor any bacteria—that dissolves the tooth in this demonstration. Vinegar (or diet Coke) would do the same thing.

Sites of Decay

The most common site of decay is the deep and narrow fissures and pits on the biting surfaces of the back teeth. Food and bacteria get caught easily there, and the area is hard to clean. The dentist finds early decay in these areas by probing with a sharp instrument.

Children should visit their dentist before they have any decay, so these natural fissures and pits can be painlessly coated with an acid-resistant sealant that keeps out food and bacteria. It’s better for children to have their first dental visit for this, rather than for drilling and filling a tooth. The enamel of baby teeth is more susceptible to decay than the enamel of permanent teeth.

The second most common site for decay is between teeth, especially just below where they contact each other. This area is hard to clean, and plaque and debris can accumulate easily. Dentists advise using dental floss once or twice a day to clean between the teeth. Dental X-rays can detect early decay in these areas.

Sides of the teeth that face the cheeks or tongue are the least likely to decay because they’re smooth and kept clean by saliva and the continual movements of the cheeks and tongue. Also, it’s easy to clean these areas by brushing.

Later in life, the gums can recede because of periodontal disease (infection of the tissues that surround the teeth). This is why people are said to be getting long in the tooth as they age. Receding gums uncover some of the root below the enamel, making it vulnerable to decay. Tooth decay in middle age and older usually occurs in these newly exposed root surfaces and in new fissures around the edges of old dental fillings. Periodontal disease will be discussed in Chapter 14.

^{*Bulimia is an eating disorder of continual episodes of bingeing (gorging on huge amounts of food) followed by “purging” (for fear of becoming fat) by self-induced vomiting and/or use of laxatives or diuretics. Regular loss of stomach fluids can cause alkalosis (a dangerous rise in the pH of blood and tissue fluids; see Chap. 3) and, along with losses caused by laxatives and diuretics, such a severe loss of electrolytes (e.g., chloride and potassium ions) that the heart doesn’t beat normally. The gag me with a spoon line in the song Valley Girls refers to self-induced vomiting. Jane Fonda, Princess Diana, Lady Gaga, and Elton John had bulimia. Singer Karen Carpenter died of it.}

Fluoride

Besides keeping the teeth clean, another way to prevent decay is to make teeth more acidresistant by incorporating fluoride into their structure. This is done by replacing some of the enamel’s hydroxy(OH)-containing crystals [Ca₁₀(PO₄)₆(OH)₂] with harder fluoride(F)-containing crystals [Ca₁₀(PO₄)₆(F)₂].

Optimally, fluoride is incorporated when enamel and dentin form—from birth until permanent teeth erupt fully at about ages 11-13. Although some teeth mineralize before birth, there’s no proof that maternal fluoride ingestion during pregnancy helps.

Fluoride is present in all natural water supplies. The recommended level is 0.7 ppm (0.7 part fluoride per million parts of water), to give maximum protection against decay without causing the mottling of enamel that can occur in developing teeth when fluoride is excessive.* Fluoridation can reduce decay by more than half (and might also lower risk of osteoporosis by strengthening bone the same way that it strengthens teeth).

Infants who are exclusively breast fed and children who don’t get fluoridated water should get a prescription for daily fluoride drops or tablets from their physician or dentist.

Only about 60% of the U.S. population has access to an adequately fluoridated municipal water supply. Antifluoridation groups have been quite successful in alarming the public when proposals for fluoridation have come up for vote. Many people are scared when told, for example, that fluoride is a rat poison. In big doses, it’s a rat poison—and even a human poison. It’s all in the size of the dose. The anticoagulant warfarin, for example, is used in big doses to kill rats, and is used in small doses as a drug to treat people who’ve had a heart attack (Chap. 7).

When natural water supplies are excessively high in fluoride, and removing fluoride to bring the level to recommended amounts has been proposed, communities have, on occasion, voted this down as well, wanting to leave their water “natural.”

Water fluoridation has been studied for 70+ years, has been demonstrated to be safe and effective in about 60 countries, and is promoted by the World Health Organization.

Fluoride’s main benefit in preventing decay comes from its incorporation into the enamel of developing teeth, but fluoride in drinking water, toothpaste, mouth rinse, etc., also helps protect teeth in adults, mainly by promoting the remineralization of enamel. As noted earlier, tooth decay in older adults often occurs in roots newly exposed because of receding gums. Topical fluoride helps protect this area of the tooth from decay as well.

^{*Mottling is the blotching of enamel—“extra-white” spots can occur at 2 ppm fluoride; brown stains at 4 ppm. Mottling looks bad, but isn’t harmful. In fact, the discovery that mottled teeth (of people living where water was naturally high in fluoride) were very resistant to tooth decay led to the discovery of fluoride’s protective effect. (Another cause of brown stains is exposure to the antibiotic tetracycline during tooth development from before birth to about 8 years old.)}

Feeding the Bacteria

Generally speaking, the amount of sugar in the diet is directly related to the amount of tooth decay. The amount of time that sugar is available to the bacteria is also crucial. Gulping a big glass of a sugared drink is less likely to cause decay than eating a small amount of a sticky food like raisins—unless we thoroughly clean our teeth right after eating (a hard habit to instill, especially among children and adolescents).

A sugary food’s potential to cause decay is measured by changes in the acidity (pH) of plaque after eating. Foods like cookies, cake, pie, and candy cause a dramatic rise in acidity in dental plaque, and it takes over 2 hours for the pH to return to normal.

There are some ways, besides brushing, to lessen this prolonged acidity, but there are drawbacks, especially from a parent’s viewpoint. One way is to eat the sugary food at the start of the meal instead of at the end (yes, dessert first). Cheese, for example, eaten after sweets hastens the neutralization of acid in the plaque.

Another way is to chew sugarless gum for about 15 minutes, right after a sugary snack. Chewing motions of the cheeks and tongue help, as does the increased saliva. Also, sugarless gum often has sorbitol, a sugar-alcohol that doesn’t promote (and may possibly even help prevent) decay.

Tooth decay is preventable. Brush and floss daily, and cut down on the frequency and amount of sweets you eat. Good nutrition, optimal levels of fluoride during tooth development, and sealing the natural pits and fissures make teeth more resistant to decay.