Root canals 2

When we begin the root canal procedure, we anesthetize the patient with one or more shots of local anesthesia to produce profound numbness. Then, if there is enough left of the crown of the tooth above gum line, (this is not always the case) we put a tiny clamp around the tooth and slip a rubber sheet with a hole in it around the clamp. This is called a rubber dam, and it isolates the tooth from the rest of the mouth.

We do this in order to prevent dropping any of the tiny instruments we use to clean out the tooth into the patient's mouth, and to retract the tongue , lips and cheek so we have a good clear field in which to work.

Now we are ready to begin gaining access to the dental pulp. First a hole is drilled in the top of the tooth above the position where the dentist expects to find the nerve. (In front teeth the hole is made on the tongue side of the tooth so it doesn't show from the front.) The diagram to the right shows the situation we face when we first enter the tooth. The access is denoted by the gray cylinder above the pulp. The nerve may be red and alive as denoted by the red color at the tip of the pulp, or it may already be dead and draining out of the root causing abscesses as denoted by the green balls at the root tips.

The nerve in the large pulp chamber at the top of the tooth is cleaned out using a round bur on a slow speed handpiece (drill). Then the canals are entered with a series of tiny files which look from a distance like flexible pins with serrations along their lengths. These are inserted into each canal, all the way to the tip starting with a very thin little file followed by progressively fatter files until the entire mass of the nerve is removed and the sides of the root canals are made smooth and clean. In between each pass with the files, the tooth is washed (irrigated) with a dilute solution of laundry bleach (Clorox) in order to wash out the debris, and to sterilize and chemically neutralize any dead tissue that may be missed by the files. (One of the reasons for the rubber dam is to keep the bleach out of the patient's mouth.) More recently, dentists have begun using a 2% solution of chlorhexidine instead of bleach as an irrigating solution. Chlorhexidine is a powerful disinfectant and is not as caustic or as objectionable if it happens to get under the rubber dam and into the patient's mouth.

Finally, once the tooth is entirely clean internally, it is dried out with tiny paper points, and each canal is fitted with the appropriate diameter rubber cone that will entirely fill and block that canal. Each file is numbered to denote its diameter, and there is a corresponding rubber cone size to be used in canals finished to that size file. The rubber is a special form called gutta percha. It is less refined than regular rubber and is somewhat gummy, sticking to the walls of the canal and thoroughly waterproofing them. It happens to be pink in color, and is the reason the third diagram above has a pink filling under the final filling in the access hole.

The diagram to the left shows a file in the access preparation of a premolar tooth. The file is worked up and down drawing the rasps against the sides of the canal. Over time with quite a lot of elbow work, the files strip tooth structure from the inside of the root canal making it smooth and debris free. The gutta percha is usually placed into the canal along with a white creamy cement which is supposed to flow into any area where the gutta percha and files cannot reach.

After the first (master) gutta percha point is inserted to the tip of the root, more (accessory) gutta percha points are inserted beside that one in order to place pressure on the cement and force the cement into every nook and cranny, and in order to force the gutta percha against the walls of the canal thus sealing it entirely. The X-ray at the right shows a finished root canal. The arrow denotes a "lateral canal" which ended up sealed even though the files never traversed it. It has filled with cement forced into it by the lateral pressure of the accessory gutta percha points placed beside the first master point.

If you look closely at the x-ray on the right, you can see that the master gutta percha point has been inserted slightly beyond the tip of the root into bone. This is a mistake, but it is of no consequence to the success of the root canal since the gutta percha and the cement are inert and do not cause inflammation at the tip of the root even when embedded in bone.

The real anatomy of the nerve inside a tooth

When we explain how a dentist does the procedure known as a root canal, we generally show a diagram of a tooth (like the one above) with a rather idealized root anatomy. This consists of a simple pulp chamber located inside the crown portion of the tooth, and one or more straight, uncomplicated root canals leading from the pulp chamber down each root of the tooth. A glance at the image at the left shows that the reality is more complicated. The root system in any given tooth is likely to be more like a network of nerves, blood vessels and connective tissue that snake around inside the tooth. While a root canal procedure consists of removing all of the dead nerve tissue from this network, in reality, it is obvious that it would be close to impossible to traverse the horizontal components with an endodontic file. Thus, any dentist doing endodontic procedures merely does the best he can. Just as long as the tip of the roots are properly sealed, the procedure will be a success since any dead tissue remaining will be sealed inside the tooth where it is isolated from the rest of the body. Of course, this does not always happen, and some root canal treatments fail in spite of the dentist's best intentions and technique simply because the odds were stacked against him from the beginning.

Do Root canals work?

The short answer is yes, root canals work about 95% of the time. The image to the right is typical of the radiographic (x-ray) appearance of an abscessed tooth. Note the dark "balloons" at the tips of the root of the broken-down molar. These "balloons are known as granulomas. They represent soft tissue that has formed around the tips of the root to deal with the dead material inside of the root canals. As the dead material and its byproducts drain into the bone, the cells in the local area have formed "granulation tissue" which is filled with tiny blood vessels (technically, the tissue is "highly vascularized"). These blood vessels provide a steady supply of white blood cells which "eat" (phagocitize) the toxic material and keep it from getting into the general circulation where it could cause more widespread infections. Most of the time this situation is "chronic" and essentially painless, but upon occasion, when the body's defenses are directed elsewhere, such as when the patient is under the influence of other physical or psychological challenges, this situation may turn "acute" and lead to swelling, drainage and pain. Given enough antibiotics, this acute situation may eventually return to a symptomless chronic problem, only to erupt into another acute episode several months later.

The image on the left shows the same tooth about two years after its original endodontic treatment and restoration with a simple filling. As you can see, the granulation tissue at the tip of the roots has almost completely healed. Note that the disappearance of granulomas does not happen in every case, however the fact that the toxic tissue inside of the root canals has been debrided (cleaned up) and the root canals sealed, means that the situation is not likely to become acute again. The only thing that this tooth needs to complete the procedure is the placement of a crown to protect the tooth from breaking.

Why do root canals have such a bad reputation?
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