There are many circumstances in which it may be recommended that a tooth receive root canal therapy (commonly called a “root canal”). Some common reasons are trauma, very deep cavities or infections of the tooth or surrounding bone.
Once a root canal has been completed on a tooth it becomes more brittle and prone to cracking or breaking over time as it is no longer alive inside. This is similar to a tree branch – while it is attached to a tree it is quite strong and has some flexibility but once it is no longer attached to the tree it becomes weaker and easier to break.
Also, the majority of teeth that require a root canal are quite broken down and/or have a significant portion missing. This structure needs to be replaced in a way that will stand up to the demands of chewing over many years.
A crown is often recommended to cover teeth that have had root canal therapy. Placing a crown helps to protect the remaining tooth and reduce the chance that it will break in the future. This is particularly important in the case of back teeth as they sustain the majority of the forces of chewing.
The decision to crown a tooth involves many clinical factors. Your dentist can review the pros and cons of placing a crown on your tooth given it's particular condition.