How Cavities & Fillings Work
By: Dr. Jerry Gordon
This may sound a bit surprising to most people, but the large majority of cavities are completely painless. This is because the outer enamel has no nerves. It is only when the cavity enters the underlying dentin that the cavity may begin to feel sensitive.
There are very few things that most people do every single day. Into this small collection goes things like eating, sleeping and breathing. So it is amazing that, for nearly everyone in the U.S., tooth brushing falls into this category! Why, you might ask, has tooth brushing gained such tremendous importance - so much so that you have memories of tooth brushing from your earliest childhood? Because of the dreaded cavity - No one wants cavities when they visit the dentist!
Dental cavities, or caries, have plagued mankind for thousands of years. Fossilized remains of men and women from the Iron Age discovered in Warwickshire, England showed a cavity rate of only 8%. When today's Warwickshire inhabitants were compared, a remarkable high cavity rate of 48% was found. A modern diet consisting of highly processed, sugar-containing foods is the most likely culprit. The problem of dental cavities has been steadily increasing for the last four centuries in industrialized nations, and despite a recent dip due to the advent of fluoride, it continues to the present day.
Dental cavities are an infection caused by a combination of carbohydrate-containing foods and bacteria that live in our mouths. The bacteria are contained in a film that continuously forms on and around our teeth. We call this film plaque. Although there are many different types of bacteria in our mouths, only a few are associated with cavities. Some of the most common include Streptococcus mutans, Lactobacillus casei and acidophilus, and Actinomyces naeslundii. When these bacteria find carbohydrates, they eat them and produce acid. The exposure to acid causes the PH on the tooth surface to drop. Before eating, the PH in the mouth is about 6.2 to 7.0, slightly more acidic than water. As "surgery foods" (candy, sugar frosted breakfast cereals, ice cream, soda an kool-aid, etc.) and other carbohydrates are eaten, the PH drops. At a PH of 5.2 to 5.5 of below, the acid begins to dissolve the hard enamel that forms the outer coating of our teeth. Every exposure to these foods allows an acid attack on the teeth for about twenty minutes!
As the cavity progresses, it invades the softer dentin directly beneath the enamel, and encroaches on the nerve and blood supply of the tooth contained within the pulp.
Cavities attack the teeth in two main ways. The first is through the pits and fissures, which are grooves that are visible on the top biting surfaces of the back teeth (molars and premolars). The pits and fissures are thin areas of enamel that contain recesses that can trap food and plaque to form a cavity. The cavity starts from a small point of attack, and spreads widely to invade the underlying dentin.
What the dentists sees when he/she finds a cavity
A cavity detected with an X-ray
The second route of acid attack is from a smooth surface, which is between, or on the front or back of teeth. In a smooth surface cavity, the acid must travel through the entire thickness of the enamel. The area of attack is generally wide, and comes to point or converges as it enters the deeper layers of the tooth.
To understand how a cavity works, we need to have a basic understanding of the anatomy of the tooth. A tooth is composed of several layers. The outermost layer (above the gum-line) is called the enamel. Enamel is the hardest and most mineralized substance in the body. Beneath the gum-line, a substance called cementum covers the tooth roots. Under the enamel and cementum is the dentin. The dentin is about as hard as bone, and, unlike the enamel, dentin contains nerve endings. Beneath the dentin is the dental pulp. The pulp is a vascular tissue, composed of capillaries, larger blood vessels, connective tissue, nerve fibers, and cells including odontoblasts, fibroblasts, macrophages, and lymphocytes. The pulp is needed to nourish the tooth during its growth and development. After a tooth is fully mature, the only function of the pulp is to let us know if it is damaged or infected by transmitting pain.
In most dental offices, after the hygienist cleans your teeth, the dentist comes in to examine your teeth. Then, out of nowhere, he or she starts rattling off alpha-numeric jargon like, 3MOD, 5DO, 13MFD, and so on. The dentist is not looking at a bowl of alphabet soup, but, rather, using a form of dental shorthand. The numbers represent which teeth have cavities or other problems. Tooth number one is the upper right third molar or wisdom tooth, the farthest tooth back in the mouth. Tooth number sixteen is the upper left third molar. Tooth number seventeen is the lower left third molar, and tooth number thirty-two is the lower right third molar. So, teeth eight and nine are the upper front teeth, or left and right central incisors, and teeth twenty-four and twenty-five are the lower front teeth, or lower left and right central incisors.
The letter part of the code refers to different parts or surfaces of the tooth. An "M" mesial, or "D" distal, is the front or back surface of the tooth, respectively. An "O" occlusal, is the top or biting surface of a back tooth (molar or premolar), and "I" incisal, is the biting edge of front teeth (incisors and canines). A "B" buccal, is the surface of the tooth towards the cheek, and an "L" lingual, is the surface of the tooth towards the tongue. So if the dentist says number 3MOD, you'll know that you have a cavity on your upper right first molar, involving the front, top, and back parts of the tooth.
This may sound a bit surprising to most people, but the large majority of cavities are completely painless. This is because the outer enamel has no nerves. It is only when the cavity enters the underlying dentin that the cavity may begin to feel sensitive. The most common symptoms are an increased sensation to cold and/or sweet foods or beverages. A cavity is often responsible for a tooth that breaks. The cavity weakens the tooth, especially when it forms under a filling or a tooth cusp, and can easily cause a fracture when biting down.
Patients are sometimes taken off-guard when they learn that they have a few cavities but they don't have any symptoms. It is far better to treat a small cavity than to wait until they have symptoms (like pain). By the time there are symptoms, the cavity may have spread to infect the dental pulp, necessitating a root canal procedure or an extraction to eliminate the infection. Always remember that most dental problems are insidious - that is, they sneak up on you. Regular dental examinations, at least twice a year, will greatly reduce the likelihood that a dental cavity will go undetected and spread, causing pain and infecting the dental pulp.
Cavities are detected a number of ways. The most common are clinical (hands-on) and radiographic (x-ray) examinations. During a clinical exam, the dentist uses a hand held instrument called an explorer to probe the tooth surface for cavities. If the explorer "catches," that means the instrument has found a weak, acid damaged part of the tooth, a dental cavity. Dentists can also use a visual examination to detect cavities. Teeth that are discolored, usually brown or black, can sometimes indicate a dental cavity. Dental x-rays, especially check-up or bitewing x-rays, are very useful in finding cavities that are wedged between teeth, or under the gum-line. These "hidden" cavities are difficult or impossible to detect visually or with the explorer. In some cases, none of these methods are adequate, and a dentist must use a special disclosing solution to diagnose a suspicious area on a tooth.
The treatment of dental cavities goes back centuries, but it was not until 1875 that Dr. G.V. Black first described an organized approach to their treatment. His systematic methods are still used today, primarily for the placement of silver (amalgam) fillings. Dr. Black advocated removing the decayed part of the tooth, and extending the tooth preparation to include certain grooves and other surfaces. This process, "extension for prevention" is considered prudent because it provides the tooth with some protection from further decay in the future.
Most cavities discovered during a dental examination will need to be treated. In general, if a cavity has broken through the enamel and is into the underlying dentin, or is able to be probed with an explorer, it has undergone cavitation, and requires treatment. Early dental cavities that have not spread to the dentin or have undergone cavitation should not be treated, as they can be healed or re-mineralized with fluoride.
The goal of treating cavities involves two basic principals: removing the decayed portion of the tooth, and rebuilding the missing tooth structure with a filling material. The dentist usually begins the procedure with an injection of local anesthetic (xylocaine in most cases). The tooth is isolated from the rest of the mouth, and in most cases, a high-speed dental drill is needed to remove the decay and prepare the tooth for the filling. Depending on which material is used, the dentist will vary the tooth preparation accordingly. After the tooth has been prepared, a liner is often used to reduce tooth sensitivity. Common liners include gluma, copalite varnish and dycal. Dycal is a compound containing calcium hydroxide, and is used in deep cavities to stimulate the dentin to regenerate and protect the dental pulp. In deeper fillings, a base is used in addition to the liner. Common bases used under dental fillings are glass ionomer cement, and zinc phosphate cement. The main purpose of the base is to insulate the tooth from temperature changes in the mouth. The dentist and patient can then choose a number of different materials to fill the tooth, but the most common are silver (amalgam), white (resin), porcelain, or gold. These materials are layered on top of the liner or base to finish the process of rebuilding the tooth.
After a tooth has been filled, it is not unusual for the tooth to be sensitive for a day or two. In general, the deeper the filling, the more likely the tooth will have prolonged sensitivity, especially to cold food or beverages. Most fillings should be completely comfortable within two weeks. In some cases, the filling will be built up too high, and a second appointment is needed to shave down the filling to a comfortable level. If sensitivity lasts more than two weeks, it may indicate that there is a void under the filling. Prolonged discomfort may also indicate a tooth that has an infected pulp, and requires root canal therapy.
In 1997, the Food and Drug Administration approved laser use in dental hard tissues (teeth) for treatment of cavities. Different types of lasers had been used in dental applications since the early 1990's for soft tissues (gums). Dental lasers have not been widely used because of their high cost and limited applications.
A common type of laser used for treatment of dental cavities is the erbium: yttrium-aluminum-garnet (Er:YAG). The technology allows tiny layers of the tooth to be removed quietly and usually without discomfort. An injection is rarely needed. Lasers can be used for the treatment of small to medium sized cavities in adults, and has also recently been approved for children.
Unfortunately, the vast majority of dental procedures cannot be treated with the laser. Dental lasers can not remove old, broken and worn-out fillings, have difficulty removing soft dental decay (present in deep cavities), can not prepare crowns (caps), inlays, onlays, and porcelain veneers. In short, most dental treatment cannot be treated with today's dental lasers.
Every dental material used to rebuild teeth has advantages and disadvantages. Dental amalgam or silver fillings have been around for over 150 years. Amalgam is composed of silver, tin, copper, mercury, and zinc. Amalgam fillings are relatively inexpensive, durable, and time-tested. On the flip side, they are considered unaesthetic because they blacken over time and can give teeth a gray appearance, and they do not strengthen the tooth. Some people worry about the potential for mercury in dental amalgam to leak out and cause a wide variety of ailments, but research does not bear this fear out.
Composite, resin, or white fillings have been around for about two decades. Composite fillings are composed of an organic polymer known as bisphenol-A-glycidyl methacrylate (BIS-GMA), and inorganic particles such as quartz, borosilicate glass, and lithium aluminum silicate. They have the advantage of requiring a more conservative tooth preparation, (less drilling required), can have a strengthening effect on the tooth, and are very aesthetic, virtually blending in with the tooth. Composite fillings are the material of choice for repairing the front teeth. On the down side, they are more technique sensitive for the dentist to place, and are highly susceptible to decay in the future if placed improperly. They also usually cost more than an amalgam.
Porcelain is sometimes used for dental fillings called inlays. Porcelain is a non-crystalline glass composed of silicon and oxygen. It has the advantage of being highly aesthetic, and is the restoration of choice for people who place the highest value in the appearance of their teeth. Porcelain has the disadvantage of being brittle and therefore susceptible to breakage. It is also even more technique sensitive to use than composite, requires two dental visits to place the filling, and costs significantly more than amalgam or composite fillings. Porcelain can also cause accelerated wear of the opposing tooth when biting.
Gold is sometimes used for dental fillings, most commonly as an inlay. Gold is not used in its pure form, but as an alloy containing 75% gold, as well as copper, silver, platinum, palladium, and zinc. Gold is extremely durable; fairly esthetic, does not damage the opposing tooth when biting, and is very well tolerated by the gums and other intra-oral tissues. A well-done gold filling can last two to four times longer than any other dental material, and might be considered the "gold standard" for dental fillings. Gold inlays, like porcelain inlays, take two dental visits to complete and are also much more costly than amalgam or composite. They are also not nearly as aesthetic as composite or porcelain. In addition, gold inlays are fairly difficult to prepare and place - just ask any third-year or fourth-year dental student. They are usually required for graduation from dental school.
People who have reduced saliva flow due to diseases such a Sjogren Syndrome, dysfunction of their salivary glands, have undergone cancer chemotherapy or radiation, and who smoke are more likely to develop cavities. Saliva is important in fighting cavities because it can rinse away plaque and food debris, and help neutralize acid. People who have limited manual dexterity and have difficulty removing plaque from their teeth may also have a higher risk of forming cavities. Some people have naturally lower oral PH, which makes them more likely to have cavities.
The easiest way to prevent cavities is by brushing your teeth and removing plaque at least three times a day, especially after eating and before bed. Flossing at least once a day is important to remove plaque between your teeth. You should brush with a soft bristled toothbrush, and angle the bristles about 45 degrees toward the gum-line. Brush for about the length of one song on the radio (3 minutes). It's a good idea to ask your dentist or hygienist to help you with proper brushing methods.
Reducing the amount and frequency of eating sugary foods can reduce the risk of forming cavities. If you are going to drink a can of sweetened soda, for instance, it is better to drink it in one sitting, than sip it throughout the day. Better yet, drink it through a straw in one sitting, to bypass the teeth altogether. Getting to the dentist at least twice a year is critical for examinations and professional cleanings.
To reduce the incidence of cavities, use toothpaste and mouthwash that contain fluoride. Fluoride is a compound that is added to most tap water supplies, toothpastes, and mouth rinses to reduce cavities. Fluoride becomes incorporated into our teeth as they develop and makes them more resistant to decay. After our teeth are formed, fluoride can reverse the progress of early cavities, and sometimes prevent the need for corrective dental treatment.
The recent drop in the number of cavities is largely due to the addition of fluoride to our drinking water. Mass water fluoridation is the most cost-effective measure available to reduce the incidence of tooth decay. The Environmental Protection Agency has determined that the acceptable tap water concentration for fluoride is 0.7 to 1.2 parts per million. Much higher levels have been associated with chalky white discolorations of the teeth known as fluorosis.
If you live in an area that does not have fluoride or are very susceptible to cavities, your dentist can use high concentration, in-office fluoride treatments and prescribe a fluoride supplement, either as a gel, in tablets or drops. In some cases, customized trays can be used while you sleep to deliver higher doses of fluoride and help strengthen teeth to prevent cavities.
A dental procedure called sealants can also help reduce cavities on the top and sides of back teeth (occlusal, buccal and lingual surfaces). A sealant is a white resin material that blankets the tooth, protecting the vulnerable pits and fissures of the tooth. Sealants are routinely placed on children's teeth to prevent cavities on their newly developing molars. The use of sealants to prevent cavities is also a cost-effective way to reduce the incidence of cavities on adults as well. Sealants are generally not used on teeth that already have fillings.
People who have a dry mouth are at risk for developing cavities, and can have their dentist prescribe artificial saliva and mouth moisturizers as well as recommend chewing sugarless gum to stimulate saliva production. Finally, an antiseptic mouthwash containing chlorhexidine gluconate such as Peridex can also be useful in killing bacteria associated with dental caries.