The most common way for the pulp to become infected is from an untreated cavity. A cavity is formed by acid in a rather unexpected way. Inside everyone's mouth is a legion of bacteria - they are completely normal and there is nothing you can do about them. Some of these bacteria metabolize (eat) carbohydrate-containing foods or beverages and make acid as a by-product. The acid is strong enough to eat through the enamel and dentin. If left untreated, it will eventually expose the underlying pulp to bacteria inside our mouths and it gets infected.

The pulp can also get infected from trauma to the tooth. A blow to a tooth can cut off the blood supply to the tooth from our jawbone, and cause the pulp tissue to slowly die. Interestingly, a tooth that breaks within the enamel and dentin during trauma is less likely to need root canal in the future because the fracture may absorb the trauma, sparing blood flow to the tooth.

A third way a tooth can become infected is if there is long standing periodontal (gum) disease around the tooth. Bacteria from the infected gums can enter the tooth through small opening on the root surface (accessory canals) and cause a retrograde infection. Whatever way the tooth becomes infected, the pulp eventually dies, and over time, will cause a painful dental abscess within the surrounding jawbone.

A tooth that becomes sensitive to hot or cold food or beverages or hurts when biting down may indicate an infected tooth. A tooth that becomes discolored or that causes the gums to swell around a tooth may also indicate a dental infection. In some cases, a tooth will have no symptoms, but a dental exam and x-ray will reveal a tooth that requires root canal.

If a tooth has an infection of the pulp, the only options are root canal therapy or extraction. As the pulp dies, the hollow tooth becomes a reservoir for bacteria to hide from the body's immune system and any drugs that could fight the infection. In some cases a dentist will prescribe antibiotics during or after root canal therapy to kill bacteria within the jawbone and tissues surrounding the tooth, but recent research has shown that this is usually unnecessary.

Root canal therapy is a complex procedure that requires both skill and experience. The dentist numbs the area of the infected tooth with local anesthesia (usually xylocaine or bupivacaine, not novocain as was used decades ago). A clamp is placed over the tooth, and a rubber membrane (rubber dam) is spread over the clamp to isolate the tooth and prepare it for the operation.

A small hole is made through the enamel and dentin, and into the pulp. The pulp is then removed with small stainless steel files of increasing diameter. After the pulp has been removed, the inside walls of the roots are shaped, almost like a sculpture. Nickel/Titanium files that fit on a slow speed drill can aid in shaping the canals. During the procedure, fluids (irrigants) such as sodium hypochloride (bleach) and a compound containing ethylenediamine-tetraacetate (EDTA) are used to both kill remaining pulp tissue and bacteria within the roots, and remove dentin shavings produced by the files.

After the pulp is removed and the inside of the roots shaped, the canal is dried with paper cones. The canal(s) are then filled with Gutta-percha. Gutta-percha is a miraculous substance that was first introduced by Bowman in 1867. It is a purified form of Mazer Wood Trees indigenous to Indonesia and Malaysia that is combined with zinc oxide and other materials to form the rubbery filling that is placed into the tooth roots. The Gutta-percha is then cemented into the roots with a sealer that usually contains zinc oxide and eugenol. The goal of the filling procedure is to hermetically seal off the tooth against bacteria.

There are two main techniques to filling a root canal, lateral condensation and warm vertical condensation. Although research is scanty, warm vertical condensation appears to have the advantage of more completely filling the tooth roots, especially the accessory canals.

Root canal therapy is usually not painful. With the effective use of anesthesia and modern techniques, most root canal therapy can be completed in one visit, and within one hour. A tooth will be sensitive for a few days after root canal therapy, and your dentist can prescribe you medication to alleviate the pain. In the meantime, do not eat on the side of the mouth that has the root canal for a few days.

Teeth in the front of the mouth called incisors and canines usually have one root, and one nerve canal within the root. Teeth on the side of the mouth called premolars usually have one or two roots and one or two root canals. The upper back teeth (molars) usually have three roots, and three or four root canals. Lower back teeth (molars) usually have two roots and three or four root canals. Generally speaking, the more nerve canals the tooth has, the more complicated the root canal is tocomplete.

All dentists are trained to do root canals in dental school. However, skill levels and experience vary widely from dentist to dentist. An experienced general dentist can do almost all root canal therapy successfully, but some general dentists prefer to have a root canal specialist (an endodontist) perform root canal on their patients.

Root canals are successful about 90 percent of the time when they are done properly. Teeth that have had root canal can become brittle and are susceptible to fracture. In most cases, it is advisable to have a crown (cap) placed over a tooth that has had root canal to rebuild and protect it.

A tooth that has root canal can fail if some of the pulp is left inside the roots (a canal is missed), the gutta-percha does not completely seal off the tooth from bacteria, the tooth is damaged during the procedure (perforation), or the tooth fractures between the roots. In most cases, a tooth with a failing root canal will cause pain, usually when biting down.

In some cases, the root canal can be re-treated. The old gutta-percha filling is removed, the tooth is reshaped and cleaned, and then re-filled. If this is not possible, a procedure called an apicoectomy can be preformed. In an apicoectomy, the tip of the root is surgically removed, and a filling is placed over the cut root tip. If these measures fail, the tooth may have to be extracted.