When a tooth's internal/external structure is lost due to decay or trauma, and there is little or no tooth structure left to place a conventional restoration or crown, a post can be used to build a core thereby allowing the placement of a crown.
What is a post?
A post can only be placed in a root treated tooth. Some of the root filling is removed allowing for the placement of the post. A post can be prefabricated and placed directly into the prepared root canal by the dentist, or be custom made by a dental technician and then placed in the root canal by the dentist.
What is a core?
If a root treated tooth is not completely broken down it may be possible for the dentist to build it back up using filling material 'core', which can then be prepared similar to a natural tooth for placement of a crown.