Usually, when you get a toothache, it is because the tooth is badly decayed or very heavily restored (filled). When a toothache is bad enough, the nerve inside it is dead or dying, and in order to relieve the toothache or abscess, it is necessary to either extract the tooth, or to remove the nerve from the tooth. The latter choice saves the tooth, and the procedure is called a root canal. When a root canal has been performed on a tooth, we say that the tooth has been "Endodontically treated". Endodontically treated teeth have a number of characteristics that makes them different than normal "vital" teeth (teeth with live nerves inside them).
Endodontically treated teeth tend to be badly damaged above the gum line due to decay and old fillings. They also are more brittle than vital teeth, something like a dead tree branch in comparison to one that is alive. For these reasons, once the root canal has been performed, these teeth need special treatment, or they generally break a few years down the line. This is especially true of back teeth that have broad biting surfaces. They are likely to shear off because of biting forces. For this reason, simple fillings do not usually restore them for very long. These teeth need to have crowns placed in order to protect the investment in the root canal treatment.
But unfortunately, because of the very poor condition these teeth have been left in as a result of the original decay, there is often not very much tooth left above the gum line to "hang" the crown on. For this reason, we need to do a special kind of filling on the tooth that will be so firmly embedded in what does remain of the tooth that it will not dislodge later when it is prepared as a core for the crown. (A core is that part of the tooth and the fillings in it that are left over after the tooth has been prepared to receive a crown.)
The first step is to remove some of the gutta percha that was used to seal one or more of the root canals in the tooth. Then, using a series of graduated rasps, the inside of the canals are enlarged and shaped to receive a specially made metal "post" that exactly fits the hole made by the last rasp. These posts are usually pre-manufactured and come in many different shapes. The post is then cemented in the hole so that it is firmly embedded in the root of the tooth, and projects up above the level of the gums in order to help retain a filling (technically known as the core) that will be placed over it.
The posts themselves come in several different materials and various shapes. The one presented here is made of titanium and is looks a bit like a wood screw, but don't let the shape fool you. No dentist would ever actually "screw" a post shaped like this into a tooth because it would crack the root. The post is placed passively in the precisely prepared hole, and held in place with one of a number of different types of cement. Other types of posts may be made of stainless steel, carbon fiber, or even ceramic materials. Some taper, like the one pictured above while others have blunt ends and look like long, thin cylinders.
The core is really just a filling that is placed over the tooth stump and the newly placed post. It is now retained on the tooth by whatever undercuts are available on the inside of the remaining tooth stump as well as the post which is firmly cemented in the root. This filling (core) is meant to be prepared for a crown, but it may remain temporarily as a finished restoration.
Finally, the tooth and filling are prepared to receive a crown. This is done by grinding down the filling and what remains of the tooth above gum line to a form similar to the one in the schematic on the right. The "margins" of the crown preparation are usually prepared slightly below the gum line, and on tooth structure, rather than on the core material. This ensures a tight, waterproof seal around the finished crown. The prepared part of the tooth above the gum which includes both tooth structure and core (filling) material is called the preparation.
Once the tooth has been prepared, a rubber impression is taken (shown at the left) and this is sent to the lab. At the lab, the impression is poured with plaster creating a plaster model which is a duplicate of the prepared tooth above the gum line. Using this model the lab fabricates a custom crown which is then sent back to the dentist for cementation on the prepared tooth.
Not every tooth with a root canal needs a post, but every tooth with a root canal needs a core!
Looking at the image above, one can see that without the core (filling) portion of the preparation, there would not be much tooth left above the gum line to retain the crown. Even a tooth that is largely intact after the root canal treatment has a hole leading into the pulp chamber to allow access for the root canal procedure. This needs to be filled before doing the crown preparation. Otherwise, the empty space inside the preparation would leave the tooth weak and prone to breaking off after the crown is placed.
A core (without a post) in a tooth with a root canal has an advantage over an ordinary filling in a tooth without a root canal. The root canalled tooth has an empty pulp chamber which also may be filled. This pulp chamber adds substantial retention to the filling. In other words, preparing the repaired tooth for a crown generally leaves a large part of the filling intact inside the tooth making it difficult to dislodge. Teeth that are mostly intact may often be filled without placing a post inside the core for additional retention without much fear that the core will break out later.
I make the decision of whether or not to place a post in the tooth based on the amount of tooth structure that is left above the gum line. If the tooth above the gums is substantially intact with only a small access hole in the top, then a core (filling) without a post is generally sufficient. If there is a lot of tooth structure missing due to decay or for any other reason, then I usually place a post in the tooth to make sure that the core will be properly retained after the crown prep has been cut.