Cavities (dental caries) are decayed areas in the teeth, the result of a process that gradually dissolves a tooth's hard outer surface (enamel) and progresses toward the interior.
Along with the common cold and gum disease, cavities are among the most common human afflictions. If cavities are not properly treated by a dentist, they continue to enlarge. Ultimately, an untreated cavity can lead to tooth loss.
Risk Factors: There are many risk factors for cavities:
For tooth decay to develop, a tooth must be susceptible, acid-producing bacteria must be present, and nutrients must be available for the bacteria to thrive. A susceptible tooth has relatively little protective fluoride incorporated into the enamel or has pronounced pits, grooves, or fissures that retain plaque. Poor oral hygiene that allows plaque and tartar to accumulate can accelerate this process. Although the mouth contains large numbers of bacteria, only certain types generate acid, which causes decay. The most common decay-causing bacteria are Streptococcus mutans.
The nutrients that decay-causing bacteria need come from the person's diet. When infants are put to bed with a bottle, their teeth have prolonged contact with the formula or milk, which increases the likelihood of decay. Large amounts of sugar in the diet also provide food for the bacteria.
Acid in the diet (for example, in cola beverages, which contain phosphoric acid) accelerates tooth decay.
Reduced saliva flow due to drugs or disorders (such as Sjögren's syndrome) places people at greater risk of tooth decay. Older people often take drugs that reduce saliva flow, increasing their risk of cavities.
Some people have especially active decay-causing bacteria in their mouth. A parent (almost always the mother) may pass these bacteria to a child through kissing or sharing eating utensils. The bacteria flourish in the child's mouth after the first teeth come in and can then cause cavities. So, a tendency toward tooth decay that runs in families does not necessarily reflect poor oral hygiene or bad eating habits.
Gum recession also makes cavities more likely to develop because it can expose the roots of teeth. Then bacteria can access the inner layers of the tooth more easily. Gum recession makes older people prone to root cavities.
Progression of Tooth Decay:
Decay in the enamel progresses slowly. After penetrating into the second layer of the tooth—the somewhat softer, less resistant dentin—decay spreads more rapidly and moves toward the pulp, the innermost part of the tooth, which contains the nerves and blood supply. Although a cavity may take 2 or 3 years to penetrate the enamel, it can travel from the dentin to the pulp—a much greater distance—in as little as a year. Thus, root decay that starts in the dentin can destroy a lot of tooth structure in a short time.
Smooth surface decay, the most preventable and reversible type, grows the slowest. In smooth surface decay, a cavity begins as a white spot where bacteria dissolve the calcium of the enamel. Smooth surface decay between the permanent teeth usually begins between the ages of 20 and 30.
Pit and fissure decay, which usually starts during the teen years in the permanent teeth, forms in the narrow grooves on the chewing surface and on the cheek side of the back teeth. Decay at these locations progresses rapidly. Many people cannot adequately clean these cavity-prone areas because the grooves are narrower than the bristles of a toothbrush.
Root decay begins on the root surface covering (cementum) that has been exposed by receding gums, usually in people past middle age. This type of decay often results from difficulty cleaning the root areas, a lack of adequate saliva flow, a diet high in sugar, or a combination of these factors. Root decay can be the most difficult type of tooth decay to prevent and treat.
Whether tooth decay causes pain depends on which part of the tooth is affected and how deeply the decay extends. A cavity in the enamel causes no pain. The pain starts when the decay reaches the dentin. People may feel pain only when drinking something cold or eating candy. This indicates that the pulp is still healthy. If the cavity is treated at this stage, dentists can restore the tooth, and most likely no further pain or chewing difficulties will develop.
A cavity that gets close to or actually reaches the pulp causes irreversible damage. Pain lingers even after a stimulus (such as cold water) is removed. The tooth may hurt even without stimulation (spontaneous toothache).
If irreversible damage to the pulp occurs and the pulp subsequently dies, the pain may stop temporarily. The tooth then may become sensitive when people bite or when the tongue or a finger presses on it because the area at the end of the root has become inflamed or because infection has developed at the root. Infection may produce a collection of pus (abscess—see Tooth Disorders: Periapical Abscess), which causes constant pain that is worse when people bite.
If a cavity is treated before it starts to hurt, the chance of damage to the pulp is reduced, and more of the tooth structure is saved. To detect cavities early, a dentist inquires about pain, examines the teeth, probes the teeth with dental instruments, and may take x-rays. People should have a dental examination every 6 to 12 months. Not every examination includes x-rays. Depending on the dentist's assessment of a person's teeth, x-rays may be taken every 12 to 36 months.
Several general strategies are key to preventing cavities:
Good oral hygiene, which involves brushing before or after breakfast and before bedtime and flossing daily to remove plaque, can effectively control smooth surface decay. Brushing helps prevent cavities from forming on the top and sides of the teeth, and flossing gets between the teeth where a brush cannot reach.
Electric and ultrasonic toothbrushes are excellent, but an ordinary toothbrush, used properly, is quite sufficient. Normally, proper brushing takes only about 3 minutes. Floss is gently moved back and forth between the teeth, then wrapped around the tooth and root surfaces in a “C” shape at the gum line. When the floss is moved with a vertical sliding motion, it can remove plaque and food debris.
Initially, plaque is quite soft, and removing it with a soft-bristled toothbrush and dental floss at least once every 24 hours makes decay unlikely. Once plaque begins to harden, a process that begins after about 72 hours, removing it becomes more difficult.
Although all carbohydrates can cause tooth decay to some degree, the biggest culprits are sugars. All simple sugars, including table sugar (sucrose) and the sugars in honey (levulose and dextrose), fruit (fructose), and milk (lactose), have the same effect on the teeth. Whenever sugar comes in contact with plaque, Streptococcus mutans bacteria in the plaque produce acid. The amount of sugar eaten is of little consequence. The amount of time the sugar stays in contact with the teeth is what matters. Thus, sipping a sugary soft drink over an hour is more damaging than eating a candy bar in 5 minutes, even though the candy bar may contain more sugar.
People who tend to develop cavities should eat sweet snacks less often. Rinsing the mouth after eating a snack removes some of the sugar, but brushing the teeth is more effective. Drinking artificially sweetened soft drinks also helps, although diet colas contain acid that can promote tooth decay. Drinking tea or coffee without sugar also can help people avoid cavities, particularly on exposed root surfaces.
Fluoride can make the teeth, particularly the enamel, more resistant to the acid that helps cause cavities. Fluoride taken internally is effective while the teeth are growing and hardening—until about age 11. Water fluoridation is the most efficient way to supply children with fluoride, and over half of the United States population now has drinking water with enough fluoride to reduce tooth decay. However, if a water supply has too much fluoride, the teeth can become spotted or discolored (fluorosis). If a child's water supply does not have enough fluoride, doctors or dentists can prescribe sodium fluoride drops or tablets. Dentists may apply fluoride directly to the teeth of people of any age if they are prone to tooth decay. Fluoridated toothpaste and concentrated mouth rinses containing fluoride are beneficial for adults as well as children.
Sealants protect hard-to-reach pits and fissures (grooves), particularly on the back teeth. After thoroughly cleaning the area to be sealed, dentists roughen the enamel with an acid solution to help the sealant adhere to the teeth. Dentists then place a liquid plastic in and over the pits and fissures of the teeth. When the liquid hardens, it forms such an effective barrier that any bacteria inside a pit or fissure stop producing acid because food can no longer reach them. About 90% of the sealant remains after 1 year and 60% after 10 years. The occasional need for repair or replacement of sealants can be assessed at periodic dental examinations.
People who are very prone to tooth decay may need antibacterial therapy. Dentists first remove decayed areas and seal all pits and fissures in the teeth. Then dentists prescribe a powerful mouth rinse (chlorhexidine) for several weeks to kill off the bacteria in any remaining plaque. The hope is that less harmful bacteria will replace the cavity-causing bacteria. To keep bacteria under control, people may use daily home fluoride rinses and chew gum containing xylitol (a sweetener that inhibits the bacteria in plaque).
If decay is halted before it reaches the dentin, the enamel can actually repair itself (remineralization) if people use fluoride. Fluoride treatment requires use of prescription-strength fluoride-containing mouthwash. Once decay reaches the dentin, dentists drill out the decayed material inside the tooth and then fill the resulting space with a filling (restoration). Treating the decay at an early stage helps maintain the strength of the tooth and limits the chance of damage to the pulp.
Fillings are made of various materials and may be put inside the tooth or around it. Silver amalgam (a combination of mercury, silver, copper, tin, and, occasionally, zinc, palladium, or indium) is most commonly used for fillings in back teeth, where strength is important and the silver color is relatively inconspicuous. Silver amalgam is relatively inexpensive and lasts an average of 14 years. However, the amalgam can last for more than 40 years if it is carefully placed using a rubber dam and the person's oral hygiene is good. The minute amount of mercury that escapes from silver amalgam is too small to affect health. Gold fillings (inlays and onlays) are excellent but are more expensive. Also, at least two dental visits are required to permanently place them.
Composite resins and porcelain fillings are used in the front teeth, where silver would be conspicuous. Increasingly, these fillings are also being used in back teeth. Although they have the advantage of being the color of the teeth, they are more expensive than silver amalgam and may not last as long, particularly in the back teeth, which take the full force of chewing.
Glass ionomer, a tooth-colored filling, is formulated to release fluoride once in place, a benefit for people especially prone to tooth decay. Glass ionomer is also used to restore areas damaged by overzealous brushing.
Root Canal Treatment and Tooth Extraction:
When tooth decay advances far enough to permanently harm the pulp, the only way to eliminate pain is to remove the pulp by root canal (endodontic) treatment or tooth removal (extraction).
If a tooth is extracted, it should be evaluated for replacement as soon as possible. Otherwise, neighboring teeth may change position and alter the person's bite.
Bridges, Crowns, and Implants:
The replacement for an extracted tooth may be a bridge—a fixed partial denture in which teeth on either side of the missing tooth are covered with crowns—or a removable partial denture. Also, implants may be used to replace missing teeth in people who have sufficient healthy bone in their jaw. Implants are metal fixtures inserted into the jawbone. The metal is a special alloy to which bone cells can attach. After a period of time, usually 4 months, the implant becomes solid with the bone, and a post is attached. Then an artificial tooth (crown) is attached to the post. The resulting tooth can handle the force of normal chewing. Implants are considered more desirable now because they do not decay, and they offer a fixed solution to removable bridges.
A crown is a cap (restoration) that fits over a tooth. Getting a properly shaped crown usually takes two visits to the dentist, although sometimes more visits are needed. On the first visit, dentists prepare the tooth by tapering it slightly, take an impression of the prepared tooth, and put a temporary crown on it. A permanent crown is then made in a dental prosthetics laboratory, using the impression. On the next visit, the temporary crown is removed, and the final crown is permanently cemented onto the prepared tooth.
Usually, crowns are made of an alloy of gold or another metal. A porcelain coating can be used to mask the color of the metal. Crowns also may be made entirely of porcelain, although porcelain is harder and more abrasive than tooth enamel and may cause wear on the opposing tooth. Also, crowns made entirely of porcelain or similar material have a slightly greater tendency to break than do those made of metal.
Last full review/revision October 2008 by James T. Ubertalli, DMD