Tooth Anatomy
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.

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.

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.  In deeper fillings, a base is used in addition to the liner.  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 (composite 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.  If sensitivity lasts more than two months it may indicate that there is a problem with the pulp. Prolonged discomfort may also indicate a tooth that has an infected pulp, and requires root canal therapy.