A cavity is essentially a hole in the tooth that is formed as the result of decay. Often times, a cavity can be repaired through the use of drilling and filling the cavity, however in some cases a root canal might be required.
The tooth is made up of several parts, with a top layer of enamel protecting the tooth. Even though enamel is the strongest substance in the body, the unique combination of food particles, sugars, and saliva found in the mouth can easily break down enamel and expose the inner tooth. Where enamel is essentially non-feeling, the inner tooth is compromised of a number of nerves that can make it quite sensitive when exposed, especially in regards to hot and cold liquids.
In very serious cavities, the decay will eat through the enamel and eventually reach the pulp cavity of the tooth. If you were to make a cross-section of a tooth, the pulp cavity would form a very small hollow tube that runs the entire length of the tooth. This cavity is filled with dental pulp and supplied by nerves and blood vessels. The root canal extends from the pulp cavity and all teeth have at least one root canal, with some teeth, like the molars have three branches of the root.
When the pulp cavity is exposed by deep tooth decay or a chipped tooth, serious damage can be made to the exposed tooth. Occasionally, the blood vessels and exposed nerves can actually die, spreading infection from the tooth and into the jaw, in addition to causing an extremely painful abscess. Root canals are intended to help prevent bone damage and abscesses to form in these situations.
In practice, a root canal can prevent the unaffected portions of the tooth to deteriorate, prevent abscesses from forming, and preventing further damage to the pulp cavity.
A root canal is preformed by drilling deep into the tooth, until the pulp cavity is reached. This can be quite painful, so doctors will usually manage some sort of local anesthetic or, in the case of sedation dentistry, a more powerful anesthetic. To protect the other teeth from infection, a rubber material is usually placed around the tooth.
Once the pulp cavity is reached, the pulp, which is infected because of the cavity, is removed. The dentist will then clean and shape the root canal, usually placing some sort of antibiotic inside the pulp cavity to prevent reinfection from occurring. Depending on the severity of the infection, it is not uncommon for a root canal to require several different trips to the dentist office, so in those cases a temporary filling is placed in the tooth to protect it from infection.
In cases where the infection caused by the cavity is very severe and has spread into other parts of the mouth, it is often necessary to administrator antibiotics as well. It is also generally required to remove a significant portion of the top part of the tooth, called the crown, so once complete the doctor will rebuild the tooth, often placing metal supports in the crown.
While certainly no pick-nick, a root canal can effectively provide a lifetime of use and will effectively remove the infection and decay.