When a tooth gets decay (a cavity), it starts off very small, and takes quite some time to progress. If the decay is caught very early on in the process, then a very conservative, simple filling can be placed to fill in the hole that was left by the decay. This is the easiest, and cheapest way to fix the tooth. However, as the decay progresses, it destroys more and more of the tooth, and can even grow inward and reach the inner nerve canal, causing pain or infection. When it gets to this stage, the tooth will need a root canal. What a root canal accomplishes is to remove the nerve and blood supply, and rid the tooth of infection, so that there is no longer any pain associated with the decay and infection. A root canal is also sometimes necessary to perform prior to placing a post in a tooth.
Many times after a root canal has been performed, or if decay has destroyed much of a tooth, a crown (cap) will be placed over the top of the tooth. What the crown serves to do is protect the tooth so that you can chew and function on the tooth normally, and the tooth does not break or crumble under the force of chewing. As long as the crown fits properly where it comes into contact with the tooth (an area known as the “margin”), then a crown will typically last for many years. However, if the crown does not fit around the tooth very well or if a gap develops between the tooth and the crown (at the margin), then decay is able form and progress beneath the crown. The tooth underneath the crown is then softened and destroyed from the decay process. If the decay that starts around the margins of a crown is caught early on, many times the small area of decay can be removed and a small filling or “patch” can be placed and the crown can be salvaged. However, if the decay has progressed more, then the crown needs to be cut off of the tooth in order to fully remove the decay and evaluate the tooth underneath.
So, a root canal doesn’t protect against decay. It is done to remove the nerve and blood supply of a tooth and clean out infection. Moreover, crowns (over the top of a tooth) can last many years, but they will not last forever and will need to be replaced at various times, usually due to recurrent decay. If decay is discovered early, it is very easy to fix. If it is found later in the process, then it is more difficult and costly to remedy.